Iptables is used to set up, maintain, and inspect the tables of IPv4 packet filter rules in the Linux kernel. Several different
tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains.

Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches. This
is called a `target', which may be a jump to a user-defined chain in the same table.

TARGETS
A firewall rule specifies criteria for a packet and a target. If the packet does not match, the next rule in the chain is the
examined; if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined
chain or one of the special values ACCEPT, DROP, QUEUE or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet on the floor. QUEUE means to pass the packet to
userspace. (How the packet can be received by a userspace process differs by the particular queue handler. 2.4.x and 2.6.x ker-
nels up to 2.6.13 include the ip_queue queue handler. Kernels 2.6.14 and later additionally include the nfnetlink_queue queue
handler. Packets with a target of QUEUE will be sent to queue number '0' in this case. Please also see the NFQUEUE target as
described later in this man page.) RETURN means stop traversing this chain and resume at the next rule in the previous (calling)
chain. If the end of a built-in chain is reached or a rule in a built-in chain with target RETURN is matched, the target speci-
fied by the chain policy determines the fate of the packet.
TABLES

There are currently three independent tables (which tables are present at any time depends on the kernel configuration options
and which modules are present).
-t, --table table
packet matching table which the command should operate on. If the kernel is configured with
automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already there.
The tables are as follows:
filter:
This is the default table (if no -t option is passed). It contains the built-in chains INPUT (for packets destined to
local sockets), FORWARD (for packets being routed through the box), and OUTPUT (for locally-generated packets).
nat:
This table is consulted when a packet that creates a new connection is encountered. It consists of three built-ins:
PREROUTING (for altering packets as soon as they come in), OUTPUT (for altering locally-generated packets before rout-
ing), and POSTROUTING (for altering packets as they are about to go out).
mangle:
This table is used for specialized packet alteration. Until kernel 2.4.17 it had two built-in chains: PREROUTING (for
altering incoming packets before routing) and OUTPUT (for altering locally-generated packets before routing). Since
kernel 2.4.18, three other built-in chains are also supported: INPUT (for packets coming into the box itself), FORWARD
(for altering packets being routed through the box), and POSTROUTING (for altering packets as they are about to go
out).
raw:
This table is used mainly for configuring exemptions from connection tracking in combination with the NOTRACK target.
It registers at the netfilter hooks with higher priority and is thus called before ip_conntrack, or any other IP
tables. It provides the following built-in chains: PREROUTING (for packets arriving via any network interface) OUTPUT
(for packets generated by local processes)
security:
This table is used for Mandatory Access Control (MAC) networking rules, such as those enabled by the SECMARK and
CONNSECMARK targets. Mandatory Access Control is implemented by Linux Security Modules such as SELinux. The security
table is called after the filter table, allowing any Discretionary Access Control (DAC) rules in the filter table to
take effect before MAC rules. This table provides the following built-in chains: INPUT (for packets coming into the
box itself), OUTPUT (for altering locally-generated packets before routing), and FORWARD (for altering packets being
routed through the box).
OPTIONS
The options that are recognized by iptables can be divided into several different groups.
COMMANDS
These options specify the desired action to perform. Only one of them can be specified on the command line unless otherwise
stated below. For long versions of the command and option names, you need to use only enough letters to ensure that iptables can
differentiate it from all other options.
-A, --append chain rule-specification
Append one or more rules to the end of the selected chain. When the source and/or destination names resolve to more than
one address, a rule will be added for each possible address combination.
-C, --check chain rule-specification
Check whether a rule matching the specification does exist in the selected chain. This command uses the same logic as -D
to find a matching entry, but does not alter the existing iptables configuration and uses its exit code to indicate suc-
cess or failure.
-D, --delete chain rule-specification
-D, --delete chain rulenum
Delete one or more rules from the selected chain. There are two versions of this command: the rule can be specified as a
number in the chain (starting at 1 for the first rule) or a rule to match.
-I, --insert chain [rulenum] rule-specification
Insert one or more rules in the selected chain as the given rule number. So, if the rule number is 1, the rule or rules
are inserted at the head of the chain. This is also the default if no rule number is specified.
-R, --replace chain rulenum rule-specification
Replace a rule in the selected chain. If the source and/or destination names resolve to multiple addresses, the command
will fail. Rules are numbered starting at 1.
-L, --list [chain]
List all rules in the selected chain. If no chain is selected, all chains are listed. Like every other iptables command,
it applies to the specified table (filter is the default), so NAT rules get listed by
iptables -t nat -n -L
Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups. It is legal to specify
the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed. The exact output is
affected by the other arguments given. The exact rules are suppressed until you use
iptables -L -v
-S, --list-rules [chain]
Print all rules in the selected chain. If no chain is selected, all chains are printed like iptables-save. Like every
other iptables command, it applies to the specified table (filter is the default).
-F, --flush [chain]
Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting all the rules one
by one.
-Z, --zero [chain [rulenum]]
Zero the packet and byte counters in all chains, or only the given chain, or only the given rule in a chain. It is legal
to specify the -L, --list (list) option as well, to see the counters immediately before they are cleared. (See above.)
-N, --new-chain chain
Create a new user-defined chain by the given name. There must be no target of that name already.
-X, --delete-chain [chain]
Delete the optional user-defined chain specified. There must be no references to the chain. If there are, you must
delete or replace the referring rules before the chain can be deleted. The chain must be empty, i.e. not contain any
rules. If no argument is given, it will attempt to delete every non-builtin chain in the table.
-P, --policy chain target
Set the policy for the chain to the given target. See the section TARGETS for the legal targets. Only built-in (non-
user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy targets.
-E, --rename-chain old-chain new-chain
Rename the user specified chain to the user supplied name. This is cosmetic, and has no effect on the structure of the
table.
-h Help. Give a (currently very brief) description of the command syntax.
PARAMETERS
The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).
[!] -p, --protocol protocol
The protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp, udplite, icmp, esp,
ah, sctp or the special keyword "all", or it can be a numeric value, representing one of these protocols or a different
one. A protocol name from /etc/protocols is also allowed. A "!" argument before the protocol inverts the test. The num-
ber zero is equivalent to all. "all" will match with all protocols and is taken as default when this option is omitted.
[!] -s, --source address[/mask][,...]
Source specification. Address can be either a network name, a hostname, a network IP address (with /mask), or a plain IP
address. Hostnames will be resolved once only, before the rule is submitted to the kernel. Please note that specifying
any name to be resolved with a remote query such as DNS is a really bad idea. The mask can be either a network mask or a
plain number, specifying the number of 1's at the left side of the network mask. Thus, a mask of 24 is equivalent to
255.255.255.0. A "!" argument before the address specification inverts the sense of the address. The flag --src is an
alias for this option. Multiple addresses can be specified, but this will expand to multiple rules (when adding with -A),
or will cause multiple rules to be deleted (with -D).
[!] -d, --destination address[/mask][,...]
Destination specification. See the description of the -s (source) flag for a detailed description of the syntax. The
flag --dst is an alias for this option.
-j, --jump target
This specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a user-defined chain
(other than the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately,
or an extension (see EXTENSIONS below). If this option is omitted in a rule (and -g is not used), then matching the rule
will have no effect on the packet's fate, but the counters on the rule will be incremented.
-g, --goto chain
This specifies that the processing should continue in a user specified chain. Unlike the --jump option return will not
continue processing in this chain but instead in the chain that called us via --jump.
[!] -i, --in-interface name
Name of an interface via which a packet was received (only for packets entering the INPUT, FORWARD and PREROUTING chains).
When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a "+", then
any interface which begins with this name will match. If this option is omitted, any interface name will match.
[!] -o, --out-interface name
Name of an interface via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING
chains). When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a
"+", then any interface which begins with this name will match. If this option is omitted, any interface name will match.
[!] -f, --fragment
This means that the rule only refers to second and further fragments of fragmented packets. Since there is no way to tell
the source or destination ports of such a packet (or ICMP type), such a packet will not match any rules which specify
them. When the "!" argument precedes the "-f" flag, the rule will only match head fragments, or unfragmented packets.
-c, --set-counters packets bytes
This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE opera-
tions).
OTHER OPTIONS
The following additional options can be specified:
-v, --verbose
Verbose output. This option makes the list command show the interface name, the rule options (if any), and the TOS masks.
The packet and byte counters are also listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multi-
pliers respectively (but see the -x flag to change this). For appending, insertion, deletion and replacement, this causes
detailed information on the rule or rules to be printed. -v may be specified multiple times to possibly emit more detailed
debug statements.
-n, --numeric
Numeric output. IP addresses and port numbers will be printed in numeric format. By default, the program will try to
display them as host names, network names, or services (whenever applicable).
-x, --exact
Expand numbers. Display the exact value of the packet and byte counters, instead of only the rounded number in K's (mul-
tiples of 1000) M's (multiples of 1000K) or G's (multiples of 1000M). This option is only relevant for the -L command.
--line-numbers
When listing rules, add line numbers to the beginning of each rule, corresponding to that rule's position in the chain.
--modprobe=command
When adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).
MATCH EXTENSIONS
iptables can use extended packet matching modules with the -m or --match options, followed by the matching module name; after
these, various extra command line options become available, depending on the specific module. You can specify multiple extended
match modules in one line, and you can use the -h or --help options after the module has been specified to receive help specific
to that module.
If the -p or --protocol was specified and if and only if an unknown option is encountered, iptables will try load a match module
of the same name as the protocol, to try making the option available.
addrtype
This module matches packets based on their address type. Address types are used within the kernel networking stack and catego-
rize addresses into various groups. The exact definition of that group depends on the specific layer three protocol.
UNSPEC an unspecified address (i.e. 0.0.0.0)
UNICAST, LOCAL, BROADCAST, ANYCAST, MULTICAST, BLACKHOLE, UNREACHABLE ,
PROHIBIT a prohibited address
THROW FIXME
NAT FIXME
XRESOLVE
[!] --src-type type Matches if the source address is of given type
[!] --dst-type type Matches if the destination address is of given type
--limit-iface-in
The address type checking can be limited to the interface the packet is coming in. This option is only valid in the PRE-
ROUTING, INPUT and FORWARD chains. It cannot be specified with the --limit-iface-out option.
--limit-iface-out
The address type checking can be limited to the interface the packet is going out. This option is only valid in the
POSTROUTING, OUTPUT and FORWARD chains. It cannot be specified with the --limit-iface-in option.
ah
This module matches the SPIs in Authentication header of IPsec packets.
[!] --ahspi spi[:spi]
cluster
deploy gateway and back-end load-sharing clusters without the need of load-balancers.
This match requires that all the nodes see the same packets. Thus, the cluster match decides if this node has to handle a packet
given the following options:
--cluster-total-nodes num Set number of total nodes in cluster.
[!] --cluster-local-node num Set the local node number ID.
[!] --cluster-local-nodemask mask Set the local node number ID mask. use this option instead of --cluster-local-node.
--cluster-hash-seed value Set seed value of the Jenkins hash.
Example:
iptables -A PREROUTING -t mangle -i eth1 -m cluster --cluster-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed
0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth2 -m cluster --cluster-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed
0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark 0xffff -j DROP
iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark 0xffff -j DROP
And the following commands to make all nodes see the same packets:
ip maddr add 01:00:5e:00:01:01 dev eth1
ip maddr add 01:00:5e:00:01:02 dev eth2
arptables -A OUTPUT -o eth1 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:01
arptables -A INPUT -i eth1 --h-length 6 --destination-mac 01:00:5e:00:01:01 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27
arptables -A OUTPUT -o eth2 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:02
arptables -A INPUT -i eth2 --h-length 6 --destination-mac 01:00:5e:00:01:02 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27
In the case of TCP connections, pickup facility has to be disabled to avoid marking TCP ACK packets coming in the reply direction
as valid.
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
comment
Allows you to add comments (up to 256 characters) to any rule.
--comment comment
Example:
iptables -A INPUT -i eth1 -m comment --comment "my local LAN"
connbytes
Match by how many bytes or packets a connection (or one of the two flows constituting the connection) has transferred so far, or
by average bytes per packet.
The counters are 64-bit and are thus not expected to overflow ;)
The primary use is to detect long-lived downloads and mark them to be scheduled using a lower priority band in traffic control.
The transferred bytes per connection can also be viewed through `conntrack -L` and accessed via ctnetlink.
NOTE that for connections which have no accounting information, the match will always return false. The "net.netfilter.nf_con-
ntrack_acct" sysctl flag controls whether new connections will be byte/packet counted. Existing connection flows will not be
gaining/losing a/the accounting structure when be sysctl flag is flipped.
[!] --connbytes from[:to]
match packets from a connection whose packets/bytes/average packet size is more than FROM and less than TO bytes/packets.
if TO is omitted only FROM check is done. "!" is used to match packets not falling in the range.
--connbytes-dir {original|reply|both}
which packets to consider
--connbytes-mode {packets|bytes|avgpkt}
whether to check the amount of packets, number of bytes transferred or the average size (in bytes) of all packets received
so far. Note that when "both" is used together with "avgpkt", and data is going (mainly) only in one direction (for exam-
ple HTTP), the average packet size will be about half of the actual data packets.
Example:
iptables .. -m connbytes --connbytes 10000:100000 --connbytes-dir both --connbytes-mode bytes ...
connlimit
Allows you to restrict the number of parallel connections to a server per client IP address (or client address block).
--connlimit-upto n
Match if the number of existing connections is below or equal n.
--connlimit-above n
Match if the number of existing connections is above n.
--connlimit-mask prefix_length
Group hosts using the prefix length. For IPv4, this must be a number between (including) 0 and 32. For IPv6, between 0 and
128. If not specified, the maximum prefix length for the applicable protocol is used.
--connlimit-saddr
Apply the limit onto the source group. This is the default if --connlimit-daddr is not specified.
--connlimit-daddr
Apply the limit onto the destination group.
Examples:
# allow 2 telnet connections per client host
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT
# you can also match the other way around:
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-upto 2 -j ACCEPT
# limit the number of parallel HTTP requests to 16 per class C sized source network (24 bit netmask)
iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16 --connlimit-mask 24 -j REJECT
# limit the number of parallel HTTP requests to 16 for the link local network
(ipv6) ip6tables -p tcp --syn --dport 80 -s fe80::/64 -m connlimit --connlimit-above 16 --connlimit-mask 64 -j REJECT
# Limit the number of connections to a particular host:
ip6tables -p tcp --syn --dport 49152:65535 -d 2001:db8::1 -m connlimit --connlimit-above 100 -j REJECT
connmark
This module matches the netfilter mark field associated with a connection (which can be set using the CONNMARK target below).
[!] --mark value[/mask]
Matches packets in connections with the given mark value (if a mask is specified, this is logically ANDed with the mark
before the comparison).
conntrack
This module, when combined with connection tracking, allows access to the connection tracking state for this packet/connection.
[!] --ctstate statelist
statelist is a comma separated list of the connection states to match. Possible states are listed below.
[!] --ctproto l4proto
Layer-4 protocol to match (by number or name)
[!] --ctorigsrc address[/mask]
[!] --ctorigdst address[/mask]
[!] --ctreplsrc address[/mask]
[!] --ctrepldst address[/mask]
Match against original/reply source/destination address
[!] --ctorigsrcport port[:port]
[!] --ctorigdstport port[:port]
[!] --ctreplsrcport port[:port]
[!] --ctrepldstport port[:port]
Match against original/reply source/destination port (TCP/UDP/etc.) or GRE key. Matching against port ranges is only sup-
ported in kernel versions above 2.6.38.
[!] --ctstatus statelist
statuslist is a comma separated list of the connection statuses to match. Possible statuses are listed below.
[!] --ctexpire time[:time]
Match remaining lifetime in seconds against given value or range of values (inclusive)
--ctdir {ORIGINAL|REPLY}
Match packets that are flowing in the specified direction. If this flag is not specified at all, matches packets in both
directions.
States for --ctstate:
INVALID
meaning that the packet is associated with no known connection
NEW meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets
in both directions, and
ESTABLISHED
meaning that the packet is associated with a connection which has seen packets in both directions,
RELATED
meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data
transfer, or an ICMP error.
UNTRACKED
meaning that the packet is not tracked at all, which happens if you use the NOTRACK target in raw table.
SNAT A virtual state, matching if the original source address differs from the reply destination.
DNAT A virtual state, matching if the original destination differs from the reply source.
Statuses for --ctstatus:
NONE None of the below.
EXPECTED
This is an expected connection (i.e. a conntrack helper set it up)
SEEN_REPLY
Conntrack has seen packets in both directions.
ASSURED
Conntrack entry should never be early-expired.
CONFIRMED
Connection is confirmed: originating packet has left box.
cpu
[!] --cpu number
Match cpu handling this packet. cpus are numbered from 0 to NR_CPUS-1 Can be used in combination with RPS (Remote Packet
Steering) or multiqueue NICs to spread network traffic on different queues.
Example:
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j REDIRECT --to-port 8081
Available since Linux 2.6.36.
dccp
[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --dccp-types mask
Match when the DCCP packet type is one of 'mask'. 'mask' is a comma-separated list of packet types. Packet types are:
REQUEST RESPONSE DATA ACK DATAACK CLOSEREQ CLOSE RESET SYNC SYNCACK INVALID.
[!] --dccp-option number
Match if DCCP option set.
dscp
This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.
[!] --dscp value
Match against a numeric (decimal or hex) value [0-63].
[!] --dscp-class class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be converted into its
according numeric value.
ecn
This allows you to match the ECN bits of the IPv4/IPv6 and TCP header. ECN is the Explicit Congestion Notification mechanism as
specified in RFC3168
[!] --ecn-tcp-cwr
This matches if the TCP ECN CWR (Congestion Window Received) bit is set.
[!] --ecn-tcp-ece
This matches if the TCP ECN ECE (ECN Echo) bit is set.
[!] --ecn-ip-ect num
This matches a particular IPv4/IPv6 ECT (ECN-Capable Transport). You have to specify a number between `0' and `3'.
esp
This module matches the SPIs in ESP header of IPsec packets.
[!] --espspi spi[:spi]
hashlimit
hashlimit uses hash buckets to express a rate limiting match (like the limit match) for a group of connections using a single
iptables rule. Grouping can be done per-hostgroup (source and/or destination address) and/or per-port. It gives you the ability
to express "N packets per time quantum per group" (see below for some examples).
A hash limit option (--hashlimit-upto, --hashlimit-above) and --hashlimit-name are required.
--hashlimit-upto amount[/second|/minute|/hour|/day]
Match if the rate is below or equal to amount/quantum. It is specified as a number, with an optional time quantum suffix;
the default is 3/hour.
--hashlimit-above amount[/second|/minute|/hour|/day]
Match if the rate is above amount/quantum.
--hashlimit-burst amount
Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not
reached, up to this number; the default is 5.
--hashlimit-mode {srcip|srcport|dstip|dstport},...
A comma-separated list of objects to take into consideration. If no --hashlimit-mode option is given, hashlimit acts like
limit, but at the expensive of doing the hash housekeeping.
--hashlimit-srcmask prefix
When --hashlimit-mode srcip is used, all source addresses encountered will be grouped according to the given prefix length
and the so-created subnet will be subject to hashlimit. prefix must be between (inclusive) 0 and 32. Note that --hash-
limit-srcmask 0 is basically doing the same thing as not specifying srcip for --hashlimit-mode, but is technically more
expensive.
--hashlimit-dstmask prefix
Like --hashlimit-srcmask, but for destination addresses.
--hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry.
--hashlimit-htable-size buckets
The number of buckets of the hash table
--hashlimit-htable-max entries
Maximum entries in the hash.
--hashlimit-htable-expire msec
After how many milliseconds do hash entries expire.
--hashlimit-htable-gcinterval msec
How many milliseconds between garbage collection intervals.
Examples:
matching on source host
"1000 packets per second for every host in 192.168.0.0/16" => -s 192.168.0.0/16 --hashlimit-mode srcip --hashlimit-upto
1000/sec
matching on source port
"100 packets per second for every service of 192.168.1.1" => -s 192.168.1.1 --hashlimit-mode srcport --hashlimit-upto
100/sec
matching on subnet
"10000 packets per minute for every /28 subnet (groups of 8 addresses) in 10.0.0.0/8" => -s 10.0.0.8 --hashlimit-mask 28
--hashlimit-upto 10000/min
helper
This module matches packets related to a specific conntrack-helper.
[!] --helper string
Matches packets related to the specified conntrack-helper.
string can be "ftp" for packets related to a ftp-session on default port. For other ports append -portnr to the value,
ie. "ftp-2121".
Same rules apply for other conntrack-helpers.
icmp
This extension can be used if `--protocol icmp' is specified. It provides the following option:
[!] --icmp-type {type[/code]|typename}
This allows specification of the ICMP type, which can be a numeric ICMP type, type/code pair, or one of the ICMP type
names shown by the command
iptables -p icmp -h
iprange
This matches on a given arbitrary range of IP addresses.
[!] --src-range from[-to]
Match source IP in the specified range.
[!] --dst-range from[-to]
Match destination IP in the specified range.
ipvs
Match IPVS connection properties.
[!] --ipvs
packet belongs to an IPVS connection
Any of the following options implies --ipvs (even negated)
[!] --vproto protocol
VIP protocol to match; by number or name, e.g. "tcp"
[!] --vaddr address[/mask]
VIP address to match
[!] --vport port
VIP port to match; by number or name, e.g. "http"
--vdir {ORIGINAL|REPLY}
flow direction of packet
[!] --vmethod {GATE|IPIP|MASQ}
IPVS forwarding method used
[!] --vportctl port
VIP port of the controlling connection to match, e.g. 21 for FTP
length
This module matches the length of the layer-3 payload (e.g. layer-4 packet) of a packet against a specific value or range of val-
ues.
[!] --length length[:length]
limit
This module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is
reached. It can be used in combination with the LOG target to give limited logging, for example.
xt_limit has no negation support - you will have to use -m hashlimit ! --hashlimit rate in this case whilst omitting --hash-
limit-mode.
--limit rate[/second|/minute|/hour|/day]
Maximum average matching rate: specified as a number, with an optional `/second', `/minute', `/hour', or `/day' suffix;
the default is 3/hour.
--limit-burst number
Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not
reached, up to this number; the default is 5.
mac
[!] --mac-source address
Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets coming
from an Ethernet device and entering the PREROUTING, FORWARD or INPUT chains.
mark
This module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).
[!] --mark value[/mask]
Matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the mask before
the comparison).
multiport
This module matches a set of source or destination ports. Up to 15 ports can be specified. A port range (port:port) counts as
two ports. It can only be used in conjunction with -p tcp or -p udp.
[!] --source-ports,--sports port[,port|,port:port]...
Match if the source port is one of the given ports. The flag --sports is a convenient alias for this option. Multiple
ports or port ranges are separated using a comma, and a port range is specified using a colon. 53,1024:65535 would there-
fore match ports 53 and all from 1024 through 65535.
[!] --destination-ports,--dports port[,port|,port:port]...
Match if the destination port is one of the given ports. The flag --dports is a convenient alias for this option.
[!] --ports port[,port|,port:port]...
Match if either the source or destination ports are equal to one of the given ports.
nfacct
The nfacct match provides the extended accounting infrastructure for iptables. You have to use this match together with the
standalone user-space utility nfacct(8)
The only option available for this match is the following:
--nfacct-name name
This allows you to specify the existing object name that will be use for accounting the traffic that this rule-set is
matching.
To use this extension, you have to create an accounting object:
nfacct add http-traffic
Then, you have to attach it to the accounting object via iptables:
iptables -I INPUT -p tcp --sport 80 -m nfacct --nfacct-name http-traffic
iptables -I OUTPUT -p tcp --dport 80 -m nfacct --nfacct-name http-traffic
Then, you can check for the amount of traffic that the rules match:
nfacct get http-traffic
{ pkts = 00000000000000000156, bytes = 00000000000000151786 } = http-traffic;
You can obtain nfacct(8) from http://www.netfilter.org or, alternatively, from the git.netfilter.org repository.
osf
The osf module does passive operating system fingerprinting. This modules compares some data (Window Size, MSS, options and their
order, TTL, DF, and others) from packets with the SYN bit set.
[!] --genre string
Match an operating system genre by using a passive fingerprinting.
--ttl level
Do additional TTL checks on the packet to determine the operating system. level can be one of the following values:
o 0 - True IP address and fingerprint TTL comparison. This generally works for LANs.
o 1 - Check if the IP header's TTL is less than the fingerprint one. Works for globally-routable addresses.
o 2 - Do not compare the TTL at all.
--log level
Log determined genres into dmesg even if they do not match the desired one. level can be one of the following values:
o 0 - Log all matched or unknown signatures
o 1 - Log only the first one
o 2 - Log all known matched signatures
You may find something like this in syslog:
Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 -> 11.22.33.44:139 hops=3 Linux [2.5-2.6:] : 1.2.3.4:42624 ->
1.2.3.5:22 hops=4
OS fingerprints are loadable using the nfnl_osf program. To load fingerprints from a file, use:
nfnl_osf -f /usr/share/xtables/pf.os
To remove them again,
nfnl_osf -f /usr/share/xtables/pf.os -d
The fingerprint database can be downlaoded from http://www.openbsd.org/cgi-bin/cvsweb/src/etc/pf.os .
owner
This module attempts to match various characteristics of the packet creator, for locally generated packets. This match is only
valid in the OUTPUT and POSTROUTING chains. Forwarded packets do not have any socket associated with them. Packets from kernel
threads do have a socket, but usually no owner.
[!] --uid-owner username
[!] --uid-owner userid[-userid]
Matches if the packet socket's file structure (if it has one) is owned by the given user. You may also specify a numerical
UID, or an UID range.
[!] --gid-owner groupname
[!] --gid-owner groupid[-groupid]
Matches if the packet socket's file structure is owned by the given group. You may also specify a numerical GID, or a GID
range.
[!] --socket-exists
Matches if the packet is associated with a socket.
physdev
This module matches on the bridge port input and output devices enslaved to a bridge device. This module is a part of the infra-
structure that enables a transparent bridging IP firewall and is only useful for kernel versions above version 2.5.44.
[!] --physdev-in name
Name of a bridge port via which a packet is received (only for packets entering the INPUT, FORWARD and PREROUTING chains).
If the interface name ends in a "+", then any interface which begins with this name will match. If the packet didn't
arrive through a bridge device, this packet won't match this option, unless '!' is used.
[!] --physdev-out name
Name of a bridge port via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING
chains). If the interface name ends in a "+", then any interface which begins with this name will match. Note that in the
nat and mangle OUTPUT chains one cannot match on the bridge output port, however one can in the filter OUTPUT chain. If
the packet won't leave by a bridge device or if it is yet unknown what the output device will be, then the packet won't
match this option, unless '!' is used.
[!] --physdev-is-in
Matches if the packet has entered through a bridge interface.
[!] --physdev-is-out
Matches if the packet will leave through a bridge interface.
[!] --physdev-is-bridged
Matches if the packet is being bridged and therefore is not being routed. This is only useful in the FORWARD and
POSTROUTING chains.
pkttype
This module matches the link-layer packet type.
[!] --pkt-type {unicast|broadcast|multicast}
policy
This modules matches the policy used by IPsec for handling a packet.
--dir {in|out}
Used to select whether to match the policy used for decapsulation or the policy that will be used for encapsulation. in
is valid in the PREROUTING, INPUT and FORWARD chains, out is valid in the POSTROUTING, OUTPUT and FORWARD chains.
--pol {none|ipsec}
Matches if the packet is subject to IPsec processing. --pol none cannot be combined with --strict.
--strict
Selects whether to match the exact policy or match if any rule of the policy matches the given policy.
For each policy element that is to be described, one can use one or more of the following options. When --strict is in effect, at
least one must be used per element.
[!] --reqid id
Matches the reqid of the policy rule. The reqid can be specified with setkey(8) using unique:id as level.
[!] --spi spi
Matches the SPI of the SA.
[!] --proto {ah|esp|ipcomp}
Matches the encapsulation protocol.
[!] --mode {tunnel|transport}
Matches the encapsulation mode.
[!] --tunnel-src addr[/mask]
Matches the source end-point address of a tunnel mode SA. Only valid with --mode tunnel.
[!] --tunnel-dst addr[/mask]
Matches the destination end-point address of a tunnel mode SA. Only valid with --mode tunnel.
--next Start the next element in the policy specification. Can only be used with --strict.
quota
Implements network quotas by decrementing a byte counter with each packet. The condition matches until the byte counter reaches
zero. Behavior is reversed with negation (i.e. the condition does not match until the byte counter reaches zero).
[!] --quota bytes
The quota in bytes.
rateest
The rate estimator can match on estimated rates as collected by the RATEEST target. It supports matching on absolute bps/pps val-
ues, comparing two rate estimators and matching on the difference between two rate estimators.
For a better understanding of the available options, these are all possible combinations:
o rateest operator rateest-bps
o rateest operator rateest-pps
o (rateest minus rateest-bps1) operator rateest-bps2
o (rateest minus rateest-pps1) operator rateest-pps2
o rateest1 operator rateest2 rateest-bps(without rate!)
o rateest1 operator rateest2 rateest-pps(without rate!)
o (rateest1 minus rateest-bps1) operator (rateest2 minus rateest-bps2)
o (rateest1 minus rateest-pps1) operator (rateest2 minus rateest-pps2)
--rateest-delta
For each estimator (either absolute or relative mode), calculate the difference between the estimator-determined flow rate
and the static value chosen with the BPS/PPS options. If the flow rate is higher than the specified BPS/PPS, 0 will be used
instead of a negative value. In other words, "max(0, rateest#_rate - rateest#_bps)" is used.
[!] --rateest-lt
Match if rate is less than given rate/estimator.
[!] --rateest-gt
Match if rate is greater than given rate/estimator.
[!] --rateest-eq
Match if rate is equal to given rate/estimator.
In the so-called "absolute mode", only one rate estimator is used and compared against a static value, while in "relative mode",
two rate estimators are compared against another.
--rateest name
Name of the one rate estimator for absolute mode.
--rateest1 name
--rateest2 name
The names of the two rate estimators for relative mode.
--rateest-bps [value]
--rateest-pps [value]
--rateest-bps1 [value]
--rateest-bps2 [value]
--rateest-pps1 [value]
--rateest-pps2 [value]
Compare the estimator(s) by bytes or packets per second, and compare against the chosen value. See the above bullet list
for which option is to be used in which case. A unit suffix may be used - available ones are: bit, [kmgt]bit, [KMGT]ibit,
Bps, [KMGT]Bps, [KMGT]iBps.
Example: This is what can be used to route outgoing data connections from an FTP server over two lines based on the available
bandwidth at the time the data connection was started:
# Estimate outgoing rates
iptables -t mangle -A POSTROUTING -o eth0 -j RATEEST --rateest-name eth0 --rateest-interval 250ms --rateest-ewma 0.5s
iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST --rateest-name ppp0 --rateest-interval 250ms --rateest-ewma 0.5s
# Mark based on available bandwidth
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest --rateest-delta --rateest1 eth0
--rateest-bps1 2.5mbit --rateest-gt --rateest2 ppp0 --rateest-bps2 2mbit -j CONNMARK --set-mark 1
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest --rateest-delta --rateest1 ppp0
--rateest-bps1 2mbit --rateest-gt --rateest2 eth0 --rateest-bps2 2.5mbit -j CONNMARK --set-mark 2
iptables -t mangle -A balance -j CONNMARK --restore-mark
realm
This matches the routing realm. Routing realms are used in complex routing setups involving dynamic routing protocols like BGP.
[!] --realm value[/mask]
Matches a given realm number (and optionally mask). If not a number, value can be a named realm from
/etc/iproute2/rt_realms (mask can not be used in that case).
recent
Allows you to dynamically create a list of IP addresses and then match against that list in a few different ways.
For example, you can create a "badguy" list out of people attempting to connect to port 139 on your firewall and then DROP all
future packets from them without considering them.
--set, --rcheck, --update and --remove are mutually exclusive.
--name name
Specify the list to use for the commands. If no name is given then DEFAULT will be used.
[!] --set
This will add the source address of the packet to the list. If the source address is already in the list, this will update
the existing entry. This will always return success (or failure if ! is passed in).
--rsource
Match/save the source address of each packet in the recent list table. This is the default.
--rdest
Match/save the destination address of each packet in the recent list table.
[!] --rcheck
Check if the source address of the packet is currently in the list.
[!] --update
Like --rcheck, except it will update the "last seen" timestamp if it matches.
[!] --remove
Check if the source address of the packet is currently in the list and if so that address will be removed from the list
and the rule will return true. If the address is not found, false is returned.
--seconds seconds
This option must be used in conjunction with one of --rcheck or --update. When used, this will narrow the match to only
happen when the address is in the list and was seen within the last given number of seconds.
--reap This option can only be used in conjunction with --seconds. When used, this will cause entries older than the last given
number of seconds to be purged.
--hitcount hits
This option must be used in conjunction with one of --rcheck or --update. When used, this will narrow the match to only
happen when the address is in the list and packets had been received greater than or equal to the given value. This option
may be used along with --seconds to create an even narrower match requiring a certain number of hits within a specific
time frame. The maximum value for the hitcount parameter is given by the "ip_pkt_list_tot" parameter of the xt_recent ker-
nel module. Exceeding this value on the command line will cause the rule to be rejected.
--rttl This option may only be used in conjunction with one of --rcheck or --update. When used, this will narrow the match to
only happen when the address is in the list and the TTL of the current packet matches that of the packet which hit the
--set rule. This may be useful if you have problems with people faking their source address in order to DoS you via this
module by disallowing others access to your site by sending bogus packets to you.
Examples:
iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP
iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy --set -j DROP
Steve's ipt_recent website (http://snowman.net/projects/ipt_recent/) also has some examples of usage.
/proc/net/xt_recent/* are the current lists of addresses and information about each entry of each list.
Each file in /proc/net/xt_recent/ can be read from to see the current list or written two using the following commands to modify
the list:
echo +addr >/proc/net/xt_recent/DEFAULT
to add addr to the DEFAULT list
echo -addr >/proc/net/xt_recent/DEFAULT
to remove addr from the DEFAULT list
echo / >/proc/net/xt_recent/DEFAULT
to flush the DEFAULT list (remove all entries).
The module itself accepts parameters, defaults shown:
ip_list_tot=100
Number of addresses remembered per table.
ip_pkt_list_tot=20
Number of packets per address remembered.
ip_list_hash_size=0
Hash table size. 0 means to calculate it based on ip_list_tot, default: 512.
ip_list_perms=0644
Permissions for /proc/net/xt_recent/* files.
ip_list_uid=0
Numerical UID for ownership of /proc/net/xt_recent/* files.
ip_list_gid=0
Numerical GID for ownership of /proc/net/xt_recent/* files.
rpfilter
Performs a reverse path filter test on a packet. If a reply to the packet would be sent via the same interface that the packet
arrived on, the packet will match. Note that, unlike the in-kernel rp_filter, packets protected by IPSec are not treated spe-
cially. Combine this match with the policy match if you want this. Also, packets arriving via the loopback interface are always
permitted. This match can only be used in the PREROUTING chain of the raw or mangle table.
--loose
Used to specifiy that the reverse path filter test should match even if the selected output device is not the expected
one.
--validmark
Also use the packets' nfmark value when performing the reverse path route lookup.
--accept-local
This will permit packets arriving from the network with a source address that is also assigned to the local machine.
--invert This will invert the sense of the match. Instead of matching packets that passed the reverse path filter test,
match those that have failed it.
Example to log and drop packets failing the reverse path filter test:
iptables -t raw -N RPFILTER
iptables -t raw -A RPFILTER -m rpfilter -j RETURN
iptables -t raw -A RPFILTER -m limit --limit 10/minute -j NFLOG --nflog-prefix "rpfilter drop"
iptables -t raw -A RPFILTER -j DROP
iptables -t raw -A PREROUTING -j RPFILTER
Example to drop failed packets, without logging:
iptables -t raw -A RPFILTER -m rpfilter --invert -j DROP
sctp
[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --chunk-types {all|any|only} chunktype[:flags] [...]
The flag letter in upper case indicates that the flag is to match if set, in the lower case indicates to match if unset.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUTDOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK
ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE ASCONF ASCONF_ACK FORWARD_TSN
chunk type available flags
DATA I U B E i u b e
ABORT T t
SHUTDOWN_COMPLETE T t
(lowercase means flag should be "off", uppercase means "on")
Examples:
iptables -A INPUT -p sctp --dport 80 -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA,INIT -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT
set
This module matches IP sets which can be defined by ipset(8).
[!] --match-set setname flag[,flag]...
where flags are the comma separated list of src and/or dst specifications and there can be no more than six of them. Hence
the command
iptables -A FORWARD -m set --match-set test src,dst
will match packets, for which (if the set type is ipportmap) the source address and destination port pair can be found in
the specified set. If the set type of the specified set is single dimension (for example ipmap), then the command will
match packets for which the source address can be found in the specified set.
The option --match-set can be replaced by --set if that does not clash with an option of other extensions.
Use of -m set requires that ipset kernel support is provided, which, for standard kernels, is the case since Linux 2.6.39.
socket
This matches if an open socket can be found by doing a socket lookup on the packet.
--transparent
Ignore non-transparent sockets.
state
This module, when combined with connection tracking, allows access to the connection tracking state for this packet.
[!] --state state
Where state is a comma separated list of the connection states to match. Possible states are INVALID meaning that the
packet could not be identified for some reason which includes running out of memory and ICMP errors which don't correspond
to any known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in
both directions, NEW meaning that the packet has started a new connection, or otherwise associated with a connection which
has not seen packets in both directions, and RELATED meaning that the packet is starting a new connection, but is associ-
ated with an existing connection, such as an FTP data transfer, or an ICMP error. UNTRACKED meaning that the packet is
not tracked at all, which happens if you use the NOTRACK target in raw table.
statistic
This module matches packets based on some statistic condition. It supports two distinct modes settable with the --mode option.
Supported options:
--mode mode
Set the matching mode of the matching rule, supported modes are random and nth.
[!] --probability p
Set the probability for a packet to be randomly matched. It only works with the random mode. p must be within 0.0 and 1.0.
The supported granularity is in 1/2147483648th increments.
[!] --every n
Match one packet every nth packet. It works only with the nth mode (see also the --packet option).
--packet p
Set the initial counter value (0 <= p <= n-1, default 0) for the nth mode.
string
This modules matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.
--algo {bm|kmp}
Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
--from offset
Set the offset from which it starts looking for any matching. If not passed, default is 0.
--to offset
Set the offset up to which should be scanned. That is, byte offset-1 (counting from 0) is the last one that is scanned.
If not passed, default is the packet size.
[!] --string pattern
Matches the given pattern.
[!] --hex-string pattern
Matches the given pattern in hex notation.
tcp
These extensions can be used if `--protocol tcp' is specified. It provides the following options:
[!] --source-port,--sport port[:port]
Source port or port range specification. This can either be a service name or a port number. An inclusive range can also
be specified, using the format first:last. If the first port is omitted, "0" is assumed; if the last is omitted, "65535"
is assumed. If the first port is greater than the second one they will be swapped. The flag --sport is a convenient
alias for this option.
[!] --destination-port,--dport port[:port]
Destination port or port range specification. The flag --dport is a convenient alias for this option.
[!] --tcp-flags mask comp
Match when the TCP flags are as specified. The first argument mask is the flags which we should examine, written as a
comma-separated list, and the second argument comp is a comma-separated list of flags which must be set. Flags are: SYN
ACK FIN RST URG PSH ALL NONE. Hence the command
iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset.
[!] --syn
Only match TCP packets with the SYN bit set and the ACK,RST and FIN bits cleared. Such packets are used to request TCP
connection initiation; for example, blocking such packets coming in an interface will prevent incoming TCP connections,
but outgoing TCP connections will be unaffected. It is equivalent to --tcp-flags SYN,RST,ACK,FIN SYN. If the "!" flag
precedes the "--syn", the sense of the option is inverted.
[!] --tcp-option number
Match if TCP option set.
tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP header. You can only use this on TCP SYN or SYN/ACK packets,
since the MSS is only negotiated during the TCP handshake at connection startup time.
[!] --mss value[:value]
Match a given TCP MSS value or range.
time
This matches if the packet arrival time/date is within a given range. All options are optional, but are ANDed when specified. All
times are interpreted as UTC by default.
--datestart YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
--datestop YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
Only match during the given time, which must be in ISO 8601 "T" notation. The possible time range is 1970-01-01T00:00:00
to 2038-01-19T04:17:07.
If --datestart or --datestop are not specified, it will default to 1970-01-01 and 2038-01-19, respectively.
--timestart hh:mm[:ss]
--timestop hh:mm[:ss]
Only match during the given daytime. The possible time range is 00:00:00 to 23:59:59. Leading zeroes are allowed (e.g.
"06:03") and correctly interpreted as base-10.
[!] --monthdays day[,day...]
Only match on the given days of the month. Possible values are 1 to 31. Note that specifying 31 will of course not match
on months which do not have a 31st day; the same goes for 28- or 29-day February.
[!] --weekdays day[,day...]
Only match on the given weekdays. Possible values are Mon, Tue, Wed, Thu, Fri, Sat, Sun, or values from 1 to 7, respec-
tively. You may also use two-character variants (Mo, Tu, etc.).
--kerneltz
Use the kernel timezone instead of UTC to determine whether a packet meets the time regulations.
About kernel timezones: Linux keeps the system time in UTC, and always does so. On boot, system time is initialized from a ref-
erential time source. Where this time source has no timezone information, such as the x86 CMOS RTC, UTC will be assumed. If the
time source is however not in UTC, userspace should provide the correct system time and timezone to the kernel once it has the
information.
Local time is a feature on top of the (timezone independent) system time. Each process has its own idea of local time, specified
via the TZ environment variable. The kernel also has its own timezone offset variable. The TZ userspace environment variable
specifies how the UTC-based system time is displayed, e.g. when you run date(1), or what you see on your desktop clock. The TZ
string may resolve to different offsets at different dates, which is what enables the automatic time-jumping in userspace. when
DST changes. The kernel's timezone offset variable is used when it has to convert between non-UTC sources, such as FAT filesys-
tems, to UTC (since the latter is what the rest of the system uses).
The caveat with the kernel timezone is that Linux distributions may ignore to set the kernel timezone, and instead only set the
system time. Even if a particular distribution does set the timezone at boot, it is usually does not keep the kernel timezone
offset - which is what changes on DST - up to date. ntpd will not touch the kernel timezone, so running it will not resolve the
issue. As such, one may encounter a timezone that is always +0000, or one that is wrong half of the time of the year. As such,
using --kerneltz is highly discouraged.
EXAMPLES. To match on weekends, use:
-m time --weekdays Sa,Su
Or, to match (once) on a national holiday block:
-m time --datestart 2007-12-24 --datestop 2007-12-27
Since the stop time is actually inclusive, you would need the following stop time to not match the first second of the new day:
-m time --datestart 2007-01-01T17:00 --datestop 2007-01-01T23:59:59
During lunch hour:
-m time --timestart 12:30 --timestop 13:30
The fourth Friday in the month:
-m time --weekdays Fr --monthdays 22,23,24,25,26,27,28
(Note that this exploits a certain mathematical property. It is not possible to say "fourth Thursday OR fourth Friday" in one
rule. It is possible with multiple rules, though.)
tos
This module matches the 8-bit Type of Service field in the IPv4 header (i.e. including the "Precedence" bits) or the (also
8-bit) Priority field in the IPv6 header.
[!] --tos value[/mask]
Matches packets with the given TOS mark value. If a mask is specified, it is logically ANDed with the TOS mark before the
comparison.
[!] --tos symbol
You can specify a symbolic name when using the tos match for IPv4. The list of recognized TOS names can be obtained by
calling iptables with -m tos -h. Note that this implies a mask of 0x3F, i.e. all but the ECN bits.
ttl
This module matches the time to live field in the IP header.
[!] --ttl-eq ttl
Matches the given TTL value.
--ttl-gt ttl
Matches if TTL is greater than the given TTL value.
--ttl-lt ttl
Matches if TTL is less than the given TTL value.
u32
U32 tests whether quantities of up to 4 bytes extracted from a packet have specified values. The specification of what to extract
is general enough to find data at given offsets from tcp headers or payloads.
[!] --u32 tests
The argument amounts to a program in a small language described below.
tests := location "=" value | tests "&&" location "=" value
value := range | value "," range
range := number | number ":" number
a single number, n, is interpreted the same as n:n. n:m is interpreted as the range of numbers >=n and <=m.
location := number | location operator number
operator := "&" | "<>" | "@"
The operators &, <> and && mean the same as in C. The = is really a set membership operator and the value syntax describes a
set. The @ operator is what allows moving to the next header and is described further below.
There are currently some artificial implementation limits on the size of the tests:
* no more than 10 of "=" (and 9 "&&"s) in the u32 argument
* no more than 10 ranges (and 9 commas) per value
* no more than 10 numbers (and 9 operators) per location
To describe the meaning of location, imagine the following machine that interprets it. There are three registers:
A is of type char *, initially the address of the IP header
B and C are unsigned 32 bit integers, initially zero
The instructions are:
number B = number;
C = (*(A+B)<<24) + (*(A+B+1)<<16) + (*(A+B+2)<<8) + *(A+B+3)
&number C = C & number
<< number C = C << number
>> number C = C >> number
@number A = A + C; then do the instruction number
Any access of memory outside [skb->data,skb->end] causes the match to fail. Otherwise the result of the computation is the final
value of C.
Whitespace is allowed but not required in the tests. However, the characters that do occur there are likely to require shell
quoting, so it is a good idea to enclose the arguments in quotes.
Example:
match IP packets with total length >= 256
The IP header contains a total length field in bytes 2-3.
--u32 "0 & 0xFFFF = 0x100:0xFFFF"
read bytes 0-3
AND that with 0xFFFF (giving bytes 2-3), and test whether that is in the range [0x100:0xFFFF]
Example: (more realistic, hence more complicated)
match ICMP packets with icmp type 0
First test that it is an ICMP packet, true iff byte 9 (protocol) = 1
--u32 "6 & 0xFF = 1 && ...
read bytes 6-9, use & to throw away bytes 6-8 and compare the result to 1. Next test that it is not a fragment. (If so, it
might be part of such a packet but we cannot always tell.) N.B.: This test is generally needed if you want to match any-
thing beyond the IP header. The last 6 bits of byte 6 and all of byte 7 are 0 iff this is a complete packet (not a frag-
ment). Alternatively, you can allow first fragments by only testing the last 5 bits of byte 6.
... 4 & 0x3FFF = 0 && ...
Last test: the first byte past the IP header (the type) is 0. This is where we have to use the @syntax. The length of the
IP header (IHL) in 32 bit words is stored in the right half of byte 0 of the IP header itself.
... 0 >> 22 & 0x3C @ 0 >> 24 = 0"
The first 0 means read bytes 0-3, >>22 means shift that 22 bits to the right. Shifting 24 bits would give the first byte,
so only 22 bits is four times that plus a few more bits. &3C then eliminates the two extra bits on the right and the first
four bits of the first byte. For instance, if IHL=5, then the IP header is 20 (4 x 5) bytes long. In this case, bytes 0-1
are (in binary) xxxx0101 yyzzzzzz, >>22 gives the 10 bit value xxxx0101yy and &3C gives 010100. @ means to use this number
as a new offset into the packet, and read four bytes starting from there. This is the first 4 bytes of the ICMP payload,
of which byte 0 is the ICMP type. Therefore, we simply shift the value 24 to the right to throw out all but the first byte
and compare the result with 0.
Example:
TCP payload bytes 8-12 is any of 1, 2, 5 or 8
First we test that the packet is a tcp packet (similar to ICMP).
--u32 "6 & 0xFF = 6 && ...
Next, test that it is not a fragment (same as above).
... 0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 = 1,2,5,8"
0>>22&3C as above computes the number of bytes in the IP header. @ makes this the new offset into the packet, which is the
start of the TCP header. The length of the TCP header (again in 32 bit words) is the left half of byte 12 of the TCP
header. The 12>>26&3C computes this length in bytes (similar to the IP header before). "@" makes this the new offset,
which is the start of the TCP payload. Finally, 8 reads bytes 8-12 of the payload and = checks whether the result is any
of 1, 2, 5 or 8.
udp
These extensions can be used if `--protocol udp' is specified. It provides the following options:
[!] --source-port,--sport port[:port]
Source port or port range specification. See the description of the --source-port option of the TCP extension for
details.
[!] --destination-port,--dport port[:port]
Destination port or port range specification. See the description of the --destination-port option of the TCP extension
for details.
unclean
This module takes no options, but attempts to match packets which seem malformed or unusual. This is regarded as experimental.
TARGET EXTENSIONS
iptables can use extended target modules: the following are included in the standard distribution.
AUDIT
This target allows to create audit records for packets hitting the target. It can be used to record accepted, dropped, and
rejected packets. See auditd(8) for additional details.
--type {accept|drop|reject}
Set type of audit record.
Example:
iptables -N AUDIT_DROP
iptables -A AUDIT_DROP -j AUDIT --type drop
iptables -A AUDIT_DROP -j DROP
CHECKSUM
This target allows to selectively work around broken/old applications. It can only be used in the mangle table.
--checksum-fill
Compute and fill in the checksum in a packet that lacks a checksum. This is particularly useful, if you need to work
around old applications such as dhcp clients, that do not work well with checksum offloads, but don't want to disable
checksum offload in your device.
CLASSIFY
This module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).
--set-class major:minor
Set the major and minor class value. The values are always interpreted as hexadecimal even if no 0x prefix is given.
CLUSTERIP
This module allows you to configure a simple cluster of nodes that share a certain IP and MAC address without an explicit load
balancer in front of them. Connections are statically distributed between the nodes in this cluster.
--new Create a new ClusterIP. You always have to set this on the first rule for a given ClusterIP.
--hashmode mode
Specify the hashing mode. Has to be one of sourceip, sourceip-sourceport, sourceip-sourceport-destport.
--clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer multicast address
--total-nodes num
Number of total nodes within this cluster.
--local-node num
Local node number within this cluster.
--hash-init rnd
Specify the random seed used for hash initialization.
CONNMARK
This module sets the netfilter mark value associated with a connection. The mark is 32 bits wide.
--set-xmark value[/mask]
Zero out the bits given by mask and XOR value into the ctmark.
--save-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the packet mark (nfmark) to the connection mark (ctmark) using the given masks. The new nfmark value is determined as
follows:
ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask)
i.e. ctmask defines what bits to clear and nfmask what bits of the nfmark to XOR into the ctmark. ctmask and nfmask
default to 0xFFFFFFFF.
--restore-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the connection mark (ctmark) to the packet mark (nfmark) using the given masks. The new ctmark value is determined as
follows:
nfmark = (nfmark & ~nfmask) ^ (ctmark & ctmask);
i.e. nfmask defines what bits to clear and ctmask what bits of the ctmark to XOR into the nfmark. ctmask and nfmask
default to 0xFFFFFFFF.
--restore-mark is only valid in the mangle table.
The following mnemonics are available for --set-xmark:
--and-mark bits
Binary AND the ctmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits is the binary negation of bits.)
--or-mark bits
Binary OR the ctmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits
Binary XOR the ctmark with bits. (Mnemonic for --set-xmark bits/0.)
--set-mark value[/mask]
Set the connection mark. If a mask is specified then only those bits set in the mask are modified.
--save-mark [--mask mask]
Copy the nfmark to the ctmark. If a mask is specified, only those bits are copied.
--restore-mark [--mask mask]
Copy the ctmark to the nfmark. If a mask is specified, only those bits are copied. This is only valid in the mangle table.
CONNSECMARK
This module copies security markings from packets to connections (if unlabeled), and from connections back to packets (also only
if unlabeled). Typically used in conjunction with SECMARK, it is valid in the security table (for backwards compatibility with
older kernels, it is also valid in the mangle table).
--save If the packet has a security marking, copy it to the connection if the connection is not marked.
--restore
If the packet does not have a security marking, and the connection does, copy the security marking from the connection to
the packet.
CT
The CT target allows to set parameters for a packet or its associated connection. The target attaches a "template" connection
tracking entry to the packet, which is then used by the conntrack core when initializing a new ct entry. This target is thus only
valid in the "raw" table.
--notrack
Disables connection tracking for this packet.
--helper name
Use the helper identified by name for the connection. This is more flexible than loading the conntrack helper modules with
preset ports.
--ctevents event[,...]
Only generate the specified conntrack events for this connection. Possible event types are: new, related, destroy, reply,
assured, protoinfo, helper, mark (this refers to the ctmark, not nfmark), natseqinfo, secmark (ctsecmark).
--expevents event[,...]
Only generate the specified expectation events for this connection. Possible event types are: new.
--zone id
Assign this packet to zone id and only have lookups done in that zone. By default, packets have zone 0.
--timeout name
Use the timeout policy identified by name for the connection. This is provides more flexible timeout policy definition
than global timeout values available at /proc/sys/net/netfilter/nf_conntrack_*_timeout_*.
DNAT
This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called
from those chains. It specifies that the destination address of the packet should be modified (and all future packets in this
connection will also be mangled), and rules should cease being examined. It takes one type of option:
--to-destination [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new destination IP address, an inclusive range of IP addresses, and optionally, a port range
(which is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then the destination
port will never be modified. If no IP address is specified then only the destination port will be modified.
In Kernels up to 2.6.10 you can add several --to-destination options. For those kernels, if you specify more than one des-
tination address, either via an address range or multiple --to-destination options, a simple round-robin (one after
another in cycle) load balancing takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don't have the abil-
ity to NAT to multiple ranges anymore.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.22).
--persistent
Gives a client the same source-/destination-address for each connection. This supersedes the SAME target. Support for
persistent mappings is available from 2.6.29-rc2.
DSCP
This target allows to alter the value of the DSCP bits within the TOS header of the IPv4 packet. As this manipulates a packet,
it can only be used in the mangle table.
--set-dscp value
Set the DSCP field to a numerical value (can be decimal or hex)
--set-dscp-class class
Set the DSCP field to a DiffServ class.
ECN
This target allows to selectively work around known ECN blackholes. It can only be used in the mangle table.
--ecn-tcp-remove
Remove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p tcp.
IDLETIMER
This target can be used to identify when interfaces have been idle for a certain period of time. Timers are identified by labels
and are created when a rule is set with a new label. The rules also take a timeout value (in seconds) as an option. If more
than one rule uses the same timer label, the timer will be restarted whenever any of the rules get a hit. One entry for each
timer is created in sysfs. This attribute contains the timer remaining for the timer to expire. The attributes are located
under the xt_idletimer class:
/sys/class/xt_idletimer/timers/
When the timer expires, the target module sends a sysfs notification to the userspace, which can then decide what to do (eg. dis-
connect to save power).
--timeout amount
This is the time in seconds that will trigger the notification.
--label string
This is a unique identifier for the timer. The maximum length for the label string is 27 characters.
LOG
Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print some information on
all matching packets (like most IP header fields) via the kernel log (where it can be read with dmesg or syslogd(8)). This is a
"non-terminating target", i.e. rule traversal continues at the next rule. So if you want to LOG the packets you refuse, use two
separate rules with the same matching criteria, first using target LOG then DROP (or REJECT).
--log-level level
Level of logging (numeric or see syslog.conf(5)).
--log-prefix prefix
Prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs.
--log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is readable by users.
--log-tcp-options
Log options from the TCP packet header.
--log-ip-options
Log options from the IP packet header.
--log-uid
Log the userid of the process which generated the packet.
MARK
This target is used to set the Netfilter mark value associated with the packet. It can, for example, be used in conjunction with
routing based on fwmark (needs iproute2). If you plan on doing so, note that the mark needs to be set in the PREROUTING chain of
the mangle table to affect routing. The mark field is 32 bits wide.
--set-xmark value[/mask]
Zeroes out the bits given by mask and XORs value into the packet mark ("nfmark"). If mask is omitted, 0xFFFFFFFF is
assumed.
--set-mark value[/mask]
Zeroes out the bits given by mask and ORs value into the packet mark. If mask is omitted, 0xFFFFFFFF is assumed.
The following mnemonics are available:
--and-mark bits
Binary AND the nfmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits is the binary negation of bits.)
--or-mark bits
Binary OR the nfmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits
Binary XOR the nfmark with bits. (Mnemonic for --set-xmark bits/0.)
MASQUERADE
This target is only valid in the nat table, in the POSTROUTING chain. It should only be used with dynamically assigned IP
(dialup) connections: if you have a static IP address, you should use the SNAT target. Masquerading is equivalent to specifying
a mapping to the IP address of the interface the packet is going out, but also has the effect that connections are forgotten when
the interface goes down. This is the correct behavior when the next dialup is unlikely to have the same interface address (and
hence any established connections are lost anyway).
--to-ports port[-port]
This specifies a range of source ports to use, overriding the default SNAT source port-selection heuristics (see above).
This is only valid if the rule also specifies -p tcp or -p udp.
--random
Randomize source port mapping If option --random is used then port mapping will be randomized (kernel >= 2.6.21).
MIRROR
This is an experimental demonstration target which inverts the source and destination fields in the IP header and retransmits the
packet. It is only valid in the INPUT, FORWARD and PREROUTING chains, and user-defined chains which are only called from those
chains. Note that the outgoing packets are NOT seen by any packet filtering chains, connection tracking or NAT, to avoid loops
and other problems.
NETMAP
This target allows you to statically map a whole network of addresses onto another network of addresses. It can only be used
from rules in the nat table.
--to address[/mask]
Network address to map to. The resulting address will be constructed in the following way: All 'one' bits in the mask are
filled in from the new `address'. All bits that are zero in the mask are filled in from the original address.
NFLOG
This target provides logging of matching packets. When this target is set for a rule, the Linux kernel will pass the packet to
the loaded logging backend to log the packet. This is usually used in combination with nfnetlink_log as logging backend, which
will multicast the packet through a netlink socket to the specified multicast group. One or more userspace processes may sub-
scribe to the group to receive the packets. Like LOG, this is a non-terminating target, i.e. rule traversal continues at the next
rule.
--nflog-group nlgroup
The netlink group (0 - 2^16-1) to which packets are (only applicable for nfnetlink_log). The default value is 0.
--nflog-prefix prefix
A prefix string to include in the log message, up to 64 characters long, useful for distinguishing messages in the logs.
--nflog-range size
The number of bytes to be copied to userspace (only applicable for nfnetlink_log). nfnetlink_log instances may specify
their own range, this option overrides it.
--nflog-threshold size
Number of packets to queue inside the kernel before sending them to userspace (only applicable for nfnetlink_log). Higher
values result in less overhead per packet, but increase delay until the packets reach userspace. The default value is 1.
NFQUEUE
This target is an extension of the QUEUE target. As opposed to QUEUE, it allows you to put a packet into any specific queue,
identified by its 16-bit queue number. It can only be used with Kernel versions 2.6.14 or later, since it requires the
nfnetlink_queue kernel support. The queue-balance option was added in Linux 2.6.31, queue-bypass in 2.6.39.
--queue-num value
This specifies the QUEUE number to use. Valid queue numbers are 0 to 65535. The default value is 0.
--queue-balance value:value
This specifies a range of queues to use. Packets are then balanced across the given queues. This is useful for multicore
systems: start multiple instances of the userspace program on queues x, x+1, .. x+n and use "--queue-balance x:x+n".
Packets belonging to the same connection are put into the same nfqueue.
--queue-bypass
By default, if no userspace program is listening on an NFQUEUE, then all packets that are to be queued are dropped. When
this option is used, the NFQUEUE rule is silently bypassed instead. The packet will move on to the next rule.
NOTRACK
This target disables connection tracking for all packets matching that rule.
It can only be used in the raw table.
RATEEST
The RATEEST target collects statistics, performs rate estimation calculation and saves the results for later evaluation using the
rateest match.
--rateest-name name
Count matched packets into the pool referred to by name, which is freely choosable.
--rateest-interval amount{s|ms|us}
Rate measurement interval, in seconds, milliseconds or microseconds.
--rateest-ewmalog value
Rate measurement averaging time constant.
REDIRECT
This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called
from those chains. It redirects the packet to the machine itself by changing the destination IP to the primary address of the
incoming interface (locally-generated packets are mapped to the 127.0.0.1 address).
--to-ports port[-port]
This specifies a destination port or range of ports to use: without this, the destination port is never altered. This is
only valid if the rule also specifies -p tcp or -p udp.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.22).
REJECT
This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so it is a termi-
nating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains
which are only called from those chains. The following option controls the nature of the error packet returned:
--reject-with type
The type given can be icmp-net-unreachable, icmp-host-unreachable, icmp-port-unreachable, icmp-proto-unreachable,
icmp-net-prohibited, icmp-host-prohibited or icmp-admin-prohibited (*) which return the appropriate ICMP error message
(port-unreachable is the default). The option tcp-reset can be used on rules which only match the TCP protocol: this
causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur
when sending mail to broken mail hosts (which won't accept your mail otherwise).
(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT
SAME
Similar to SNAT/DNAT depending on chain: it takes a range of addresses (`--to 1.2.3.4-1.2.3.7') and gives a client the same
source-/destination-address for each connection.
N.B.: The DNAT target's --persistent option replaced the SAME target.
--to ipaddr[-ipaddr]
Addresses to map source to. May be specified more than once for multiple ranges.
--nodst
Don't use the destination-ip in the calculations when selecting the new source-ip
--random
Port mapping will be forcibly randomized to avoid attacks based on port prediction (kernel >= 2.6.21).
SECMARK
This is used to set the security mark value associated with the packet for use by security subsystems such as SELinux. It is
valid in the security table (for backwards compatibility with older kernels, it is also valid in the mangle table). The mark is
32 bits wide.
--selctx security_context
SET
This modules adds and/or deletes entries from IP sets which can be defined by ipset(8).
--add-set setname flag[,flag...]
add the address(es)/port(s) of the packet to the sets
--del-set setname flag[,flag...]
delete the address(es)/port(s) of the packet from the sets
where flags are src and/or dst specifications and there can be no more than six of them.
--timeout value
when adding entry, the timeout value to use instead of the default one from the set definition
--exist
when adding entry if it already exists, reset the timeout value to the specified one or to the default from the set defi-
nition
Use of -j SET requires that ipset kernel support is provided, which, for standard kernels, is the case since Linux 2.6.39.
SNAT
This target is only valid in the nat table, in the POSTROUTING chain. It specifies that the source address of the packet should
be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes
one type of option:
--to-source [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new source IP address, an inclusive range of IP addresses, and optionally, a port range (which
is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then source ports below 512
will be mapped to other ports below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and
other ports will be mapped to 1024 or above. Where possible, no port alteration will occur.
In Kernels up to 2.6.10, you can add several --to-source options. For those kernels, if you specify more than one source
address, either via an address range or multiple --to-source options, a simple round-robin (one after another in cycle)
takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don't have the ability to NAT to multiple ranges any-
more.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.21).
--persistent
Gives a client the same source-/destination-address for each connection. This supersedes the SAME target. Support for
persistent mappings is available from 2.6.29-rc2.
TCPMSS
This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting
it to your outgoing interface's MTU minus 40 for IPv4 or 60 for IPv6, respectively). Of course, it can only be used in conjunc-
tion with -p tcp.
This target is used to overcome criminally braindead ISPs or servers which block "ICMP Fragmentation Needed" or "ICMPv6 Packet
Too Big" packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines
behind it can never exchange large packets:
1. Web browsers connect, then hang with no data received.
2. Small mail works fine, but large emails hang.
3. ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall configuration like:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value
Explicitly sets MSS option to specified value. If the MSS of the packet is already lower than value, it will not be
increased (from Linux 2.6.25 onwards) to avoid more problems with hosts relying on a proper MSS.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40 for IPv4; -60 for IPv6). This may not function as desired where asymmet-
ric routes with differing path MTU exist -- the kernel uses the path MTU which it would use to send packets from itself to
the source and destination IP addresses. Prior to Linux 2.6.25, only the path MTU to the destination IP address was con-
sidered by this option; subsequent kernels also consider the path MTU to the source IP address.
These options are mutually exclusive.
TCPOPTSTRIP
This target will strip TCP options off a TCP packet. (It will actually replace them by NO-OPs.) As such, you will need to add the
-p tcp parameters.
--strip-options option[,option...]
Strip the given option(s). The options may be specified by TCP option number or by symbolic name. The list of recognized
options can be obtained by calling iptables with -j TCPOPTSTRIP -h.
TEE
The TEE target will clone a packet and redirect this clone to another machine on the local network segment. In other words, the
nexthop must be the target, or you will have to configure the nexthop to forward it further if so desired.
--gateway ipaddr
Send the cloned packet to the host reachable at the given IP address. Use of 0.0.0.0 (for IPv4 packets) or :: (IPv6) is
invalid.
To forward all incoming traffic on eth0 to an Network Layer logging box:
-t mangle -A PREROUTING -i eth0 -j TEE --gateway 2001:db8::1
TOS
This module sets the Type of Service field in the IPv4 header (including the "precedence" bits) or the Priority field in the IPv6
header. Note that TOS shares the same bits as DSCP and ECN. The TOS target is only valid in the mangle table.
--set-tos value[/mask]
Zeroes out the bits given by mask (see NOTE below) and XORs value into the TOS/Priority field. If mask is omitted, 0xFF is
assumed.
--set-tos symbol
You can specify a symbolic name when using the TOS target for IPv4. It implies a mask of 0xFF (see NOTE below). The list
of recognized TOS names can be obtained by calling iptables with -j TOS -h.
The following mnemonics are available:
--and-tos bits
Binary AND the TOS value with bits. (Mnemonic for --set-tos 0/invbits, where invbits is the binary negation of bits. See
NOTE below.)
--or-tos bits
Binary OR the TOS value with bits. (Mnemonic for --set-tos bits/bits. See NOTE below.)
--xor-tos bits
Binary XOR the TOS value with bits. (Mnemonic for --set-tos bits/0. See NOTE below.)
NOTE: In Linux kernels up to and including 2.6.38, with the exception of longterm releases 2.6.32 (>=.42), 2.6.33 (>=.15), and
2.6.35 (>=.14), there is a bug whereby IPv6 TOS mangling does not behave as documented and differs from the IPv4 version. The TOS
mask indicates the bits one wants to zero out, so it needs to be inverted before applying it to the original TOS field. However,
the aformentioned kernels forgo the inversion which breaks --set-tos and its mnemonics.
TPROXY
This target is only valid in the mangle table, in the PREROUTING chain and user-defined chains which are only called from this
chain. It redirects the packet to a local socket without changing the packet header in any way. It can also change the mark value
which can then be used in advanced routing rules. It takes three options:
--on-port port
This specifies a destination port to use. It is a required option, 0 means the new destination port is the same as the
original. This is only valid if the rule also specifies -p tcp or -p udp.
--on-ip address
This specifies a destination address to use. By default the address is the IP address of the incoming interface. This is
only valid if the rule also specifies -p tcp or -p udp.
--tproxy-mark value[/mask]
Marks packets with the given value/mask. The fwmark value set here can be used by advanced routing. (Required for trans-
parent proxying to work: otherwise these packets will get forwarded, which is probably not what you want.)
TRACE
This target marks packets so that the kernel will log every rule which match the packets as those traverse the tables, chains,
rules.
A logging backend, such as ip(6)t_LOG or nfnetlink_log, must be loaded for this to be visible. The packets are logged with the
string prefix: "TRACE: tablename:chainname:type:rulenum " where type can be "rule" for plain rule, "return" for implicit rule at
the end of a user defined chain and "policy" for the policy of the built in chains.
It can only be used in the raw table.
TTL
This is used to modify the IPv4 TTL header field. The TTL field determines how many hops (routers) a packet can traverse until
it's time to live is exceeded.
Setting or incrementing the TTL field can potentially be very dangerous, so it should be avoided at any cost. This target is only
valid in mangle table.
Don't ever set or increment the value on packets that leave your local network!
--ttl-set value
Set the TTL value to `value'.
--ttl-dec value
Decrement the TTL value `value' times.
--ttl-inc value
Increment the TTL value `value' times.
ULOG
This target provides userspace logging of matching packets. When this target is set for a rule, the Linux kernel will multicast
this packet through a netlink socket. One or more userspace processes may then subscribe to various multicast groups and receive
the packets. Like LOG, this is a "non-terminating target", i.e. rule traversal continues at the next rule.
--ulog-nlgroup nlgroup
This specifies the netlink group (1-32) to which the packet is sent. Default value is 1.
--ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32 characters long, and useful for distinguishing messages in the
logs.
--ulog-cprange size
Number of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. Default
is 0.
--ulog-qthreshold size
Number of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets inside the kernel and
transmits them as one netlink multipart message to userspace. Default is 1 (for backwards compatibility).
DIAGNOSTICS
Various error messages are printed to standard error. The exit code is 0 for correct functioning. Errors which appear to be
caused by invalid or abused command line parameters cause an exit code of 2, and other errors cause an exit code of 1.
BUGS
Bugs? What's this? ;-) Well, you might want to have a look at http://bugzilla.netfilter.org/
COMPATIBILITY WITH IPCHAINS
This iptables is very similar to ipchains by Rusty Russell. The main difference is that the chains INPUT and OUTPUT are only
traversed for packets coming into the local host and originating from the local host respectively. Hence every packet only
passes through one of the three chains (except loopback traffic, which involves both INPUT and OUTPUT chains); previously a for-
warded packet would pass through all three.
The other main difference is that -i refers to the input interface; -o refers to the output interface, and both are available for
packets entering the FORWARD chain.
The various forms of NAT have been separated out; iptables is a pure packet filter when using the default `filter' table, with
optional extension modules. This should simplify much of the previous confusion over the combination of IP masquerading and
packet filtering seen previously. So the following options are handled differently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
SEE ALSO
iptables-save(8), iptables-restore(8), ip6tables(8), ip6tables-save(8), ip6tables-restore(8), libipq(3).
The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO
details the extensions that are not in the standard distribution, and the netfilter-hacking-HOWTO details the netfilter inter-
nals.
See http://www.netfilter.org/.
AUTHORS
Rusty Russell originally wrote iptables, in early consultation with Michael Neuling.
Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection framework in iptables, then wrote the mangle
table, the owner match, the mark stuff, and ran around doing cool stuff everywhere.
James Morris wrote the TOS target, and tos match.
Jozsef Kadlecsik wrote the REJECT target.
Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as well as the TTL, DSCP, ECN matches and targets.
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Yasuyuki Kozakai, Jozsef Kadlecsik, Patrick McHardy, James Morris,
Pablo Neira Ayuso, Harald Welte and Rusty Russell.
Man page originally written by Herve Eychenne .
VERSION
This manual page applies to iptables @PACKAGE_VERSION@.
9 minutes
good bye from .bash_logout
Connection to real-world-systems.com closed.
DATA/dgerman >iptables
-bash: iptables: command not found
DATA/dgerman >ssh dapie
dgerman@dapie's password:
I am .profile , v1.26 called from -bash from:PPID 14933. I am the big one
Mon Oct 9 17:15:38 EDT 2017
Timezone now:
Timezone now: US/Eastern
Mon Oct 9 17:15:38 EDT 2017
DaPie
DaPie:/home/dgerman > iptables
iptables v1.4.14: no command specified
Try `iptables -h' or 'iptables --help' for more information.
DaPie:/home/dgerman > man iptables
IPTABLES(8) iptables 1.4.14 IPTABLES(8)
NAME
iptables -- administration tool for IPv4 packet filtering and NAT
SYNOPSIS
iptables [-t table] {-A|-C|-D} chain rule-specification
iptables [-t table] -I chain [rulenum] rule-specification
iptables [-t table] -R chain rulenum rule-specification
iptables [-t table] -D chain rulenum
iptables [-t table] -S [chain [rulenum]]
iptables [-t table] {-F|-L|-Z} [chain [rulenum]] [options...]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target
iptables [-t table] -E old-chain-name new-chain-name
rule-specification = [matches...] [target]
match = -m matchname [per-match-options]
target = -j targetname [per-target-options]
DESCRIPTION
Iptables is used to set up, maintain, and inspect the tables of IPv4 packet filter rules in the Linux kernel. Several different
tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains.
Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches. This
is called a `target', which may be a jump to a user-defined chain in the same table.
TARGETS
A firewall rule specifies criteria for a packet and a target. If the packet does not match, the next rule in the chain is the
examined; if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined
chain or one of the special values ACCEPT, DROP, QUEUE or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet on the floor. QUEUE means to pass the packet to
userspace. (How the packet can be received by a userspace process differs by the particular queue handler. 2.4.x and 2.6.x ker-
nels up to 2.6.13 include the ip_queue queue handler. Kernels 2.6.14 and later additionally include the nfnetlink_queue queue
handler. Packets with a target of QUEUE will be sent to queue number '0' in this case. Please also see the NFQUEUE target as
described later in this man page.) RETURN means stop traversing this chain and resume at the next rule in the previous (calling)
chain. If the end of a built-in chain is reached or a rule in a built-in chain with target RETURN is matched, the target speci-
fied by the chain policy determines the fate of the packet.
TABLES
There are currently three independent tables (which tables are present at any time depends on the kernel configuration options
and which modules are present).
-t, --table table
packet matching table which the command should operate on. If the kernel is configured with
automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already
there.
The tables are as follows:
filter:
This is the default table (if no -t option is passed). It contains the built-in chains INPUT (for packets destined to
local sockets), FORWARD (for packets being routed through the box), and OUTPUT (for locally-generated packets).
nat:
This table is consulted when a packet that creates a new connection is encountered. It consists of three built-ins:
PREROUTING (for altering packets as soon as they come in), OUTPUT (for altering locally-generated packets before rout-
ing), and POSTROUTING (for altering packets as they are about to go out).
mangle:
This table is used for specialized packet alteration. Until kernel 2.4.17 it had two built-in chains: PREROUTING (for
altering incoming packets before routing) and OUTPUT (for altering locally-generated packets before routing). Since
kernel 2.4.18, three other built-in chains are also supported: INPUT (for packets coming into the box itself), FORWARD
(for altering packets being routed through the box), and POSTROUTING (for altering packets as they are about to go
out).
raw:
This table is used mainly for configuring exemptions from connection tracking in combination with the NOTRACK target.
It registers at the netfilter hooks with higher priority and is thus called before ip_conntrack, or any other IP
tables. It provides the following built-in chains: PREROUTING (for packets arriving via any network interface) OUTPUT
(for packets generated by local processes)
security:
This table is used for Mandatory Access Control (MAC) networking rules, such as those enabled by the SECMARK and
CONNSECMARK targets. Mandatory Access Control is implemented by Linux Security Modules such as SELinux. The security
table is called after the filter table, allowing any Discretionary Access Control (DAC) rules in the filter table to
take effect before MAC rules. This table provides the following built-in chains: INPUT (for packets coming into the
box itself), OUTPUT (for altering locally-generated packets before routing), and FORWARD (for altering packets being
routed through the box).
OPTIONS
The options that are recognized by iptables can be divided into several different groups.
COMMANDS
These options specify the desired action to perform. Only one of them can be specified on the command line unless otherwise
stated below. For long versions of the command and option names, you need to use only enough letters to ensure that iptables can
differentiate it from all other options.
-A, --append chain rule-specification
Append one or more rules to the end of the selected chain. When the source and/or destination names resolve to more than
one address, a rule will be added for each possible address combination.
-C, --check chain rule-specification
Check whether a rule matching the specification does exist in the selected chain. This command uses the same logic as -D
to find a matching entry, but does not alter the existing iptables configuration and uses its exit code to indicate suc-
cess or failure.
-D, --delete chain rule-specification
-D, --delete chain rulenum
Delete one or more rules from the selected chain. There are two versions of this command: the rule can be specified as a
number in the chain (starting at 1 for the first rule) or a rule to match.
-I, --insert chain [rulenum] rule-specification
Insert one or more rules in the selected chain as the given rule number. So, if the rule number is 1, the rule or rules
are inserted at the head of the chain. This is also the default if no rule number is specified.
-R, --replace chain rulenum rule-specification
Replace a rule in the selected chain. If the source and/or destination names resolve to multiple addresses, the command
will fail. Rules are numbered starting at 1.
-L, --list [chain]
List all rules in the selected chain. If no chain is selected, all chains are listed. Like every other iptables command,
it applies to the specified table (filter is the default), so NAT rules get listed by
iptables -t nat -n -L
Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups. It is legal to specify
the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed. The exact output is
affected by the other arguments given. The exact rules are suppressed until you use
iptables -L -v
-S, --list-rules [chain]
Print all rules in the selected chain. If no chain is selected, all chains are printed like iptables-save. Like every
other iptables command, it applies to the specified table (filter is the default).
-F, --flush [chain]
Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting all the rules one
by one.
-Z, --zero [chain [rulenum]]
Zero the packet and byte counters in all chains, or only the given chain, or only the given rule in a chain. It is legal
to specify the -L, --list (list) option as well, to see the counters immediately before they are cleared. (See above.)
-N, --new-chain chain
Create a new user-defined chain by the given name. There must be no target of that name already.
-X, --delete-chain [chain]
Delete the optional user-defined chain specified. There must be no references to the chain. If there are, you must
delete or replace the referring rules before the chain can be deleted. The chain must be empty, i.e. not contain any
rules. If no argument is given, it will attempt to delete every non-builtin chain in the table.
-P, --policy chain target
Set the policy for the chain to the given target. See the section TARGETS for the legal targets. Only built-in (non-
user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy targets.
-E, --rename-chain old-chain new-chain
Rename the user specified chain to the user supplied name. This is cosmetic, and has no effect on the structure of the
table.
-h Help. Give a (currently very brief) description of the command syntax.
PARAMETERS
The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).
[!] -p, --protocol protocol
The protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp, udplite, icmp, esp,
ah, sctp or the special keyword "all", or it can be a numeric value, representing one of these protocols or a different
one. A protocol name from /etc/protocols is also allowed. A "!" argument before the protocol inverts the test. The num-
ber zero is equivalent to all. "all" will match with all protocols and is taken as default when this option is omitted.
[!] -s, --source address[/mask][,...]
Source specification. Address can be either a network name, a hostname, a network IP address (with /mask), or a plain IP
address. Hostnames will be resolved once only, before the rule is submitted to the kernel. Please note that specifying
any name to be resolved with a remote query such as DNS is a really bad idea. The mask can be either a network mask or a
plain number, specifying the number of 1's at the left side of the network mask. Thus, a mask of 24 is equivalent to
255.255.255.0. A "!" argument before the address specification inverts the sense of the address. The flag --src is an
alias for this option. Multiple addresses can be specified, but this will expand to multiple rules (when adding with -A),
or will cause multiple rules to be deleted (with -D).
[!] -d, --destination address[/mask][,...]
Destination specification. See the description of the -s (source) flag for a detailed description of the syntax. The
flag --dst is an alias for this option.
-j, --jump target
This specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a user-defined chain
(other than the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately,
or an extension (see EXTENSIONS below). If this option is omitted in a rule (and -g is not used), then matching the rule
will have no effect on the packet's fate, but the counters on the rule will be incremented.
-g, --goto chain
This specifies that the processing should continue in a user specified chain. Unlike the --jump option return will not
continue processing in this chain but instead in the chain that called us via --jump.
[!] -i, --in-interface name
Name of an interface via which a packet was received (only for packets entering the INPUT, FORWARD and PREROUTING chains).
When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a "+", then
any interface which begins with this name will match. If this option is omitted, any interface name will match.
[!] -o, --out-interface name
Name of an interface via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING
chains). When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a
"+", then any interface which begins with this name will match. If this option is omitted, any interface name will match.
[!] -f, --fragment
This means that the rule only refers to second and further fragments of fragmented packets. Since there is no way to tell
the source or destination ports of such a packet (or ICMP type), such a packet will not match any rules which specify
them. When the "!" argument precedes the "-f" flag, the rule will only match head fragments, or unfragmented packets.
-c, --set-counters packets bytes
This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE opera-
tions).
OTHER OPTIONS
The following additional options can be specified:
-v, --verbose
Verbose output. This option makes the list command show the interface name, the rule options (if any), and the TOS masks.
The packet and byte counters are also listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multi-
pliers respectively (but see the -x flag to change this). For appending, insertion, deletion and replacement, this causes
detailed information on the rule or rules to be printed. -v may be specified multiple times to possibly emit more detailed
debug statements.
-n, --numeric
Numeric output. IP addresses and port numbers will be printed in numeric format. By default, the program will try to
display them as host names, network names, or services (whenever applicable).
-x, --exact
Expand numbers. Display the exact value of the packet and byte counters, instead of only the rounded number in K's (mul-
tiples of 1000) M's (multiples of 1000K) or G's (multiples of 1000M). This option is only relevant for the -L command.
--line-numbers
When listing rules, add line numbers to the beginning of each rule, corresponding to that rule's position in the chain.
--modprobe=command
When adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).
MATCH EXTENSIONS
iptables can use extended packet matching modules with the -m or --match options, followed by the matching module name; after
these, various extra command line options become available, depending on the specific module. You can specify multiple extended
match modules in one line, and you can use the -h or --help options after the module has been specified to receive help specific
to that module.
If the -p or --protocol was specified and if and only if an unknown option is encountered, iptables will try load a match module
of the same name as the protocol, to try making the option available.
addrtype
This module matches packets based on their address type. Address types are used within the kernel networking stack and catego-
rize addresses into various groups. The exact definition of that group depends on the specific layer three protocol.
The following address types are possible:
UNSPEC an unspecified address (i.e. 0.0.0.0)
UNICAST
an unicast address
LOCAL a local address
BROADCAST
a broadcast address
ANYCAST
an anycast packet
MULTICAST
a multicast address
BLACKHOLE
a blackhole address
UNREACHABLE
an unreachable address
PROHIBIT
a prohibited address
THROW FIXME
NAT FIXME
XRESOLVE
[!] --src-type type
Matches if the source address is of given type
[!] --dst-type type
Matches if the destination address is of given type
--limit-iface-in
The address type checking can be limited to the interface the packet is coming in. This option is only valid in the PRE-
ROUTING, INPUT and FORWARD chains. It cannot be specified with the --limit-iface-out option.
--limit-iface-out
The address type checking can be limited to the interface the packet is going out. This option is only valid in the
POSTROUTING, OUTPUT and FORWARD chains. It cannot be specified with the --limit-iface-in option.
ah
This module matches the SPIs in Authentication header of IPsec packets.
[!] --ahspi spi[:spi]
cluster
Allows you to deploy gateway and back-end load-sharing clusters without the need of load-balancers.
This match requires that all the nodes see the same packets. Thus, the cluster match decides if this node has to handle a packet
given the following options:
--cluster-total-nodes num
Set number of total nodes in cluster.
[!] --cluster-local-node num
Set the local node number ID.
[!] --cluster-local-nodemask mask
Set the local node number ID mask. You can use this option instead of --cluster-local-node.
--cluster-hash-seed value
Set seed value of the Jenkins hash.
Example:
iptables -A PREROUTING -t mangle -i eth1 -m cluster --cluster-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed
0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth2 -m cluster --cluster-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed
0xdeadbeef -j MARK --set-mark 0xffff
iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark 0xffff -j DROP
iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark 0xffff -j DROP
And the following commands to make all nodes see the same packets:
ip maddr add 01:00:5e:00:01:01 dev eth1
ip maddr add 01:00:5e:00:01:02 dev eth2
arptables -A OUTPUT -o eth1 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:01
arptables -A INPUT -i eth1 --h-length 6 --destination-mac 01:00:5e:00:01:01 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27
arptables -A OUTPUT -o eth2 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:02
arptables -A INPUT -i eth2 --h-length 6 --destination-mac 01:00:5e:00:01:02 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27
In the case of TCP connections, pickup facility has to be disabled to avoid marking TCP ACK packets coming in the reply direction
as valid.
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
comment
Allows you to add comments (up to 256 characters) to any rule.
--comment comment
Example:
iptables -A INPUT -i eth1 -m comment --comment "my local LAN"
connbytes
Match by how many bytes or packets a connection (or one of the two flows constituting the connection) has transferred so far, or
by average bytes per packet.
The counters are 64-bit and are thus not expected to overflow ;)
The primary use is to detect long-lived downloads and mark them to be scheduled using a lower priority band in traffic control.
The transferred bytes per connection can also be viewed through `conntrack -L` and accessed via ctnetlink.
NOTE that for connections which have no accounting information, the match will always return false. The "net.netfilter.nf_con-
ntrack_acct" sysctl flag controls whether new connections will be byte/packet counted. Existing connection flows will not be
gaining/losing a/the accounting structure when be sysctl flag is flipped.
[!] --connbytes from[:to]
match packets from a connection whose packets/bytes/average packet size is more than FROM and less than TO bytes/packets.
if TO is omitted only FROM check is done. "!" is used to match packets not falling in the range.
--connbytes-dir {original|reply|both}
which packets to consider
--connbytes-mode {packets|bytes|avgpkt}
whether to check the amount of packets, number of bytes transferred or the average size (in bytes) of all packets received
so far. Note that when "both" is used together with "avgpkt", and data is going (mainly) only in one direction (for exam-
ple HTTP), the average packet size will be about half of the actual data packets.
Example:
iptables .. -m connbytes --connbytes 10000:100000 --connbytes-dir both --connbytes-mode bytes ...
connlimit
Allows you to restrict the number of parallel connections to a server per client IP address (or client address block).
--connlimit-upto n
Match if the number of existing connections is below or equal n.
--connlimit-above n
Match if the number of existing connections is above n.
--connlimit-mask prefix_length
Group hosts using the prefix length. For IPv4, this must be a number between (including) 0 and 32. For IPv6, between 0 and
128. If not specified, the maximum prefix length for the applicable protocol is used.
--connlimit-saddr
Apply the limit onto the source group. This is the default if --connlimit-daddr is not specified.
--connlimit-daddr
Apply the limit onto the destination group.
Examples:
# allow 2 telnet connections per client host
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT
# you can also match the other way around:
iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-upto 2 -j ACCEPT
# limit the number of parallel HTTP requests to 16 per class C sized source network (24 bit netmask)
iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16 --connlimit-mask 24 -j REJECT
# limit the number of parallel HTTP requests to 16 for the link local network
(ipv6) ip6tables -p tcp --syn --dport 80 -s fe80::/64 -m connlimit --connlimit-above 16 --connlimit-mask 64 -j REJECT
# Limit the number of connections to a particular host:
ip6tables -p tcp --syn --dport 49152:65535 -d 2001:db8::1 -m connlimit --connlimit-above 100 -j REJECT
connmark
This module matches the netfilter mark field associated with a connection (which can be set using the CONNMARK target below).
[!] --mark value[/mask]
Matches packets in connections with the given mark value (if a mask is specified, this is logically ANDed with the mark
before the comparison).
conntrack
This module, when combined with connection tracking, allows access to the connection tracking state for this packet/connection.
[!] --ctstate statelist
statelist is a comma separated list of the connection states to match. Possible states are listed below.
[!] --ctproto l4proto
Layer-4 protocol to match (by number or name)
[!] --ctorigsrc address[/mask]
[!] --ctorigdst address[/mask]
[!] --ctreplsrc address[/mask]
[!] --ctrepldst address[/mask]
Match against original/reply source/destination address
[!] --ctorigsrcport port[:port]
[!] --ctorigdstport port[:port]
[!] --ctreplsrcport port[:port]
[!] --ctrepldstport port[:port]
Match against original/reply source/destination port (TCP/UDP/etc.) or GRE key. Matching against port ranges is only sup-
ported in kernel versions above 2.6.38.
[!] --ctstatus statelist
statuslist is a comma separated list of the connection statuses to match. Possible statuses are listed below.
[!] --ctexpire time[:time]
Match remaining lifetime in seconds against given value or range of values (inclusive)
--ctdir {ORIGINAL|REPLY}
Match packets that are flowing in the specified direction. If this flag is not specified at all, matches packets in both
directions.
States for --ctstate:
INVALID
meaning that the packet is associated with no known connection
NEW meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets
in both directions, and
ESTABLISHED
meaning that the packet is associated with a connection which has seen packets in both directions,
RELATED
meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data
transfer, or an ICMP error.
UNTRACKED
meaning that the packet is not tracked at all, which happens if you use the NOTRACK target in raw table.
SNAT A virtual state, matching if the original source address differs from the reply destination.
DNAT A virtual state, matching if the original destination differs from the reply source.
Statuses for --ctstatus:
NONE None of the below.
EXPECTED
This is an expected connection (i.e. a conntrack helper set it up)
SEEN_REPLY
Conntrack has seen packets in both directions.
ASSURED
Conntrack entry should never be early-expired.
CONFIRMED
Connection is confirmed: originating packet has left box.
cpu
[!] --cpu number
Match cpu handling this packet. cpus are numbered from 0 to NR_CPUS-1 Can be used in combination with RPS (Remote Packet
Steering) or multiqueue NICs to spread network traffic on different queues.
Example:
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j REDIRECT --to-port 8081
Available since Linux 2.6.36.
dccp
[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --dccp-types mask
Match when the DCCP packet type is one of 'mask'. 'mask' is a comma-separated list of packet types. Packet types are:
REQUEST RESPONSE DATA ACK DATAACK CLOSEREQ CLOSE RESET SYNC SYNCACK INVALID.
[!] --dccp-option number
Match if DCCP option set.
dscp
This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.
[!] --dscp value
Match against a numeric (decimal or hex) value [0-63].
[!] --dscp-class class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be converted into its
according numeric value.
ecn
This allows you to match the ECN bits of the IPv4/IPv6 and TCP header. ECN is the Explicit Congestion Notification mechanism as
specified in RFC3168
[!] --ecn-tcp-cwr
This matches if the TCP ECN CWR (Congestion Window Received) bit is set.
[!] --ecn-tcp-ece
This matches if the TCP ECN ECE (ECN Echo) bit is set.
[!] --ecn-ip-ect num
This matches a particular IPv4/IPv6 ECT (ECN-Capable Transport). You have to specify a number between `0' and `3'.
esp
This module matches the SPIs in ESP header of IPsec packets.
[!] --espspi spi[:spi]
hashlimit
hashlimit uses hash buckets to express a rate limiting match (like the limit match) for a group of connections using a single
iptables rule. Grouping can be done per-hostgroup (source and/or destination address) and/or per-port. It gives you the ability
to express "N packets per time quantum per group" (see below for some examples).
A hash limit option (--hashlimit-upto, --hashlimit-above) and --hashlimit-name are required.
--hashlimit-upto amount[/second|/minute|/hour|/day]
Match if the rate is below or equal to amount/quantum. It is specified as a number, with an optional time quantum suffix;
the default is 3/hour.
--hashlimit-above amount[/second|/minute|/hour|/day]
Match if the rate is above amount/quantum.
--hashlimit-burst amount
Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not
reached, up to this number; the default is 5.
--hashlimit-mode {srcip|srcport|dstip|dstport},...
A comma-separated list of objects to take into consideration. If no --hashlimit-mode option is given, hashlimit acts like
limit, but at the expensive of doing the hash housekeeping.
--hashlimit-srcmask prefix
When --hashlimit-mode srcip is used, all source addresses encountered will be grouped according to the given prefix length
and the so-created subnet will be subject to hashlimit. prefix must be between (inclusive) 0 and 32. Note that --hash-
limit-srcmask 0 is basically doing the same thing as not specifying srcip for --hashlimit-mode, but is technically more
expensive.
--hashlimit-dstmask prefix
Like --hashlimit-srcmask, but for destination addresses.
--hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry.
--hashlimit-htable-size buckets
The number of buckets of the hash table
--hashlimit-htable-max entries
Maximum entries in the hash.
--hashlimit-htable-expire msec
After how many milliseconds do hash entries expire.
--hashlimit-htable-gcinterval msec
How many milliseconds between garbage collection intervals.
Examples:
matching on source host
"1000 packets per second for every host in 192.168.0.0/16" => -s 192.168.0.0/16 --hashlimit-mode srcip --hashlimit-upto
1000/sec
matching on source port
"100 packets per second for every service of 192.168.1.1" => -s 192.168.1.1 --hashlimit-mode srcport --hashlimit-upto
100/sec
matching on subnet
"10000 packets per minute for every /28 subnet (groups of 8 addresses) in 10.0.0.0/8" => -s 10.0.0.8 --hashlimit-mask 28
--hashlimit-upto 10000/min
helper
This module matches packets related to a specific conntrack-helper.
[!] --helper string
Matches packets related to the specified conntrack-helper.
string can be "ftp" for packets related to a ftp-session on default port. For other ports append -portnr to the value,
ie. "ftp-2121".
Same rules apply for other conntrack-helpers.
icmp
This extension can be used if `--protocol icmp' is specified. It provides the following option:
[!] --icmp-type {type[/code]|typename}
This allows specification of the ICMP type, which can be a numeric ICMP type, type/code pair, or one of the ICMP type
names shown by the command
iptables -p icmp -h
iprange
This matches on a given arbitrary range of IP addresses.
[!] --src-range from[-to]
Match source IP in the specified range.
[!] --dst-range from[-to]
Match destination IP in the specified range.
ipvs
Match IPVS connection properties.
[!] --ipvs
packet belongs to an IPVS connection
Any of the following options implies --ipvs (even negated)
[!] --vproto protocol
VIP protocol to match; by number or name, e.g. "tcp"
[!] --vaddr address[/mask]
VIP address to match
[!] --vport port
VIP port to match; by number or name, e.g. "http"
--vdir {ORIGINAL|REPLY}
flow direction of packet
[!] --vmethod {GATE|IPIP|MASQ}
IPVS forwarding method used
[!] --vportctl port
VIP port of the controlling connection to match, e.g. 21 for FTP
length
This module matches the length of the layer-3 payload (e.g. layer-4 packet) of a packet against a specific value or range of val-
ues.
[!] --length length[:length]
limit
This module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is
reached. It can be used in combination with the LOG target to give limited logging, for example.
xt_limit has no negation support - you will have to use -m hashlimit ! --hashlimit rate in this case whilst omitting --hash-
limit-mode.
--limit rate[/second|/minute|/hour|/day]
Maximum average matching rate: specified as a number, with an optional `/second', `/minute', `/hour', or `/day' suffix;
the default is 3/hour.
--limit-burst number
Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not
reached, up to this number; the default is 5.
mac
[!] --mac-source address
Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets coming
from an Ethernet device and entering the PREROUTING, FORWARD or INPUT chains.
mark
This module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).
[!] --mark value[/mask]
Matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the mask before
the comparison).
multiport
This module matches a set of source or destination ports. Up to 15 ports can be specified. A port range (port:port) counts as
two ports. It can only be used in conjunction with -p tcp or -p udp.
[!] --source-ports,--sports port[,port|,port:port]...
Match if the source port is one of the given ports. The flag --sports is a convenient alias for this option. Multiple
ports or port ranges are separated using a comma, and a port range is specified using a colon. 53,1024:65535 would there-
fore match ports 53 and all from 1024 through 65535.
[!] --destination-ports,--dports port[,port|,port:port]...
Match if the destination port is one of the given ports. The flag --dports is a convenient alias for this option.
[!] --ports port[,port|,port:port]...
Match if either the source or destination ports are equal to one of the given ports.
nfacct
The nfacct match provides the extended accounting infrastructure for iptables. You have to use this match together with the
standalone user-space utility nfacct(8)
The only option available for this match is the following:
--nfacct-name name
This allows you to specify the existing object name that will be use for accounting the traffic that this rule-set is
matching.
To use this extension, you have to create an accounting object:
nfacct add http-traffic
Then, you have to attach it to the accounting object via iptables:
iptables -I INPUT -p tcp --sport 80 -m nfacct --nfacct-name http-traffic
iptables -I OUTPUT -p tcp --dport 80 -m nfacct --nfacct-name http-traffic
Then, you can check for the amount of traffic that the rules match:
nfacct get http-traffic
{ pkts = 00000000000000000156, bytes = 00000000000000151786 } = http-traffic;
You can obtain nfacct(8) from http://www.netfilter.org or, alternatively, from the git.netfilter.org repository.
osf
The osf module does passive operating system fingerprinting. This modules compares some data (Window Size, MSS, options and their
order, TTL, DF, and others) from packets with the SYN bit set.
[!] --genre string
Match an operating system genre by using a passive fingerprinting.
--ttl level
Do additional TTL checks on the packet to determine the operating system. level can be one of the following values:
o 0 - True IP address and fingerprint TTL comparison. This generally works for LANs.
o 1 - Check if the IP header's TTL is less than the fingerprint one. Works for globally-routable addresses.
o 2 - Do not compare the TTL at all.
--log level
Log determined genres into dmesg even if they do not match the desired one. level can be one of the following values:
o 0 - Log all matched or unknown signatures
o 1 - Log only the first one
o 2 - Log all known matched signatures
You may find something like this in syslog:
Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 -> 11.22.33.44:139 hops=3 Linux [2.5-2.6:] : 1.2.3.4:42624 ->
1.2.3.5:22 hops=4
OS fingerprints are loadable using the nfnl_osf program. To load fingerprints from a file, use:
nfnl_osf -f /usr/share/xtables/pf.os
To remove them again,
nfnl_osf -f /usr/share/xtables/pf.os -d
The fingerprint database can be downlaoded from http://www.openbsd.org/cgi-bin/cvsweb/src/etc/pf.os .
owner
This module attempts to match various characteristics of the packet creator, for locally generated packets. This match is only
valid in the OUTPUT and POSTROUTING chains. Forwarded packets do not have any socket associated with them. Packets from kernel
threads do have a socket, but usually no owner.
[!] --uid-owner username
[!] --uid-owner userid[-userid]
Matches if the packet socket's file structure (if it has one) is owned by the given user. You may also specify a numerical
UID, or an UID range.
[!] --gid-owner groupname
[!] --gid-owner groupid[-groupid]
Matches if the packet socket's file structure is owned by the given group. You may also specify a numerical GID, or a GID
range.
[!] --socket-exists
Matches if the packet is associated with a socket.
physdev
This module matches on the bridge port input and output devices enslaved to a bridge device. This module is a part of the infra-
structure that enables a transparent bridging IP firewall and is only useful for kernel versions above version 2.5.44.
[!] --physdev-in name
Name of a bridge port via which a packet is received (only for packets entering the INPUT, FORWARD and PREROUTING chains).
If the interface name ends in a "+", then any interface which begins with this name will match. If the packet didn't
arrive through a bridge device, this packet won't match this option, unless '!' is used.
[!] --physdev-out name
Name of a bridge port via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING
chains). If the interface name ends in a "+", then any interface which begins with this name will match. Note that in the
nat and mangle OUTPUT chains one cannot match on the bridge output port, however one can in the filter OUTPUT chain. If
the packet won't leave by a bridge device or if it is yet unknown what the output device will be, then the packet won't
match this option, unless '!' is used.
[!] --physdev-is-in
Matches if the packet has entered through a bridge interface.
[!] --physdev-is-out
Matches if the packet will leave through a bridge interface.
[!] --physdev-is-bridged
Matches if the packet is being bridged and therefore is not being routed. This is only useful in the FORWARD and
POSTROUTING chains.
pkttype
This module matches the link-layer packet type.
[!] --pkt-type {unicast|broadcast|multicast}
policy
This modules matches the policy used by IPsec for handling a packet.
--dir {in|out}
Used to select whether to match the policy used for decapsulation or the policy that will be used for encapsulation. in
is valid in the PREROUTING, INPUT and FORWARD chains, out is valid in the POSTROUTING, OUTPUT and FORWARD chains.
--pol {none|ipsec}
Matches if the packet is subject to IPsec processing. --pol none cannot be combined with --strict.
--strict
Selects whether to match the exact policy or match if any rule of the policy matches the given policy.
For each policy element that is to be described, one can use one or more of the following options. When --strict is in effect, at
least one must be used per element.
[!] --reqid id
Matches the reqid of the policy rule. The reqid can be specified with setkey(8) using unique:id as level.
[!] --spi spi
Matches the SPI of the SA.
[!] --proto {ah|esp|ipcomp}
Matches the encapsulation protocol.
[!] --mode {tunnel|transport}
Matches the encapsulation mode.
[!] --tunnel-src addr[/mask]
Matches the source end-point address of a tunnel mode SA. Only valid with --mode tunnel.
[!] --tunnel-dst addr[/mask]
Matches the destination end-point address of a tunnel mode SA. Only valid with --mode tunnel.
--next Start the next element in the policy specification. Can only be used with --strict.
quota
Implements network quotas by decrementing a byte counter with each packet. The condition matches until the byte counter reaches
zero. Behavior is reversed with negation (i.e. the condition does not match until the byte counter reaches zero).
[!] --quota bytes
The quota in bytes.
rateest
The rate estimator can match on estimated rates as collected by the RATEEST target. It supports matching on absolute bps/pps val-
ues, comparing two rate estimators and matching on the difference between two rate estimators.
For a better understanding of the available options, these are all possible combinations:
o rateest operator rateest-bps
o rateest operator rateest-pps
o (rateest minus rateest-bps1) operator rateest-bps2
o (rateest minus rateest-pps1) operator rateest-pps2
o rateest1 operator rateest2 rateest-bps(without rate!)
o rateest1 operator rateest2 rateest-pps(without rate!)
o (rateest1 minus rateest-bps1) operator (rateest2 minus rateest-bps2)
o (rateest1 minus rateest-pps1) operator (rateest2 minus rateest-pps2)
--rateest-delta
For each estimator (either absolute or relative mode), calculate the difference between the estimator-determined flow rate
and the static value chosen with the BPS/PPS options. If the flow rate is higher than the specified BPS/PPS, 0 will be used
instead of a negative value. In other words, "max(0, rateest#_rate - rateest#_bps)" is used.
[!] --rateest-lt
Match if rate is less than given rate/estimator.
[!] --rateest-gt
Match if rate is greater than given rate/estimator.
[!] --rateest-eq
Match if rate is equal to given rate/estimator.
In the so-called "absolute mode", only one rate estimator is used and compared against a static value, while in "relative mode",
two rate estimators are compared against another.
--rateest name
Name of the one rate estimator for absolute mode.
--rateest1 name
--rateest2 name
The names of the two rate estimators for relative mode.
--rateest-bps [value]
--rateest-pps [value]
--rateest-bps1 [value]
--rateest-bps2 [value]
--rateest-pps1 [value]
--rateest-pps2 [value]
Compare the estimator(s) by bytes or packets per second, and compare against the chosen value. See the above bullet list
for which option is to be used in which case. A unit suffix may be used - available ones are: bit, [kmgt]bit, [KMGT]ibit,
Bps, [KMGT]Bps, [KMGT]iBps.
Example: This is what can be used to route outgoing data connections from an FTP server over two lines based on the available
bandwidth at the time the data connection was started:
# Estimate outgoing rates
iptables -t mangle -A POSTROUTING -o eth0 -j RATEEST --rateest-name eth0 --rateest-interval 250ms --rateest-ewma 0.5s
iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST --rateest-name ppp0 --rateest-interval 250ms --rateest-ewma 0.5s
# Mark based on available bandwidth
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest --rateest-delta --rateest1 eth0
--rateest-bps1 2.5mbit --rateest-gt --rateest2 ppp0 --rateest-bps2 2mbit -j CONNMARK --set-mark 1
iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest --rateest-delta --rateest1 ppp0
--rateest-bps1 2mbit --rateest-gt --rateest2 eth0 --rateest-bps2 2.5mbit -j CONNMARK --set-mark 2
iptables -t mangle -A balance -j CONNMARK --restore-mark
realm
This matches the routing realm. Routing realms are used in complex routing setups involving dynamic routing protocols like BGP.
[!] --realm value[/mask]
Matches a given realm number (and optionally mask). If not a number, value can be a named realm from
/etc/iproute2/rt_realms (mask can not be used in that case).
recent
Allows you to dynamically create a list of IP addresses and then match against that list in a few different ways.
For example, you can create a "badguy" list out of people attempting to connect to port 139 on your firewall and then DROP all
future packets from them without considering them.
--set, --rcheck, --update and --remove are mutually exclusive.
--name name
Specify the list to use for the commands. If no name is given then DEFAULT will be used.
[!] --set
This will add the source address of the packet to the list. If the source address is already in the list, this will update
the existing entry. This will always return success (or failure if ! is passed in).
--rsource
Match/save the source address of each packet in the recent list table. This is the default.
--rdest
Match/save the destination address of each packet in the recent list table.
[!] --rcheck
Check if the source address of the packet is currently in the list.
[!] --update
Like --rcheck, except it will update the "last seen" timestamp if it matches.
[!] --remove
Check if the source address of the packet is currently in the list and if so that address will be removed from the list
and the rule will return true. If the address is not found, false is returned.
--seconds seconds
This option must be used in conjunction with one of --rcheck or --update. When used, this will narrow the match to only
happen when the address is in the list and was seen within the last given number of seconds.
--reap This option can only be used in conjunction with --seconds. When used, this will cause entries older than the last given
number of seconds to be purged.
--hitcount hits
This option must be used in conjunction with one of --rcheck or --update. When used, this will narrow the match to only
happen when the address is in the list and packets had been received greater than or equal to the given value. This option
may be used along with --seconds to create an even narrower match requiring a certain number of hits within a specific
time frame. The maximum value for the hitcount parameter is given by the "ip_pkt_list_tot" parameter of the xt_recent ker-
nel module. Exceeding this value on the command line will cause the rule to be rejected.
--rttl This option may only be used in conjunction with one of --rcheck or --update. When used, this will narrow the match to
only happen when the address is in the list and the TTL of the current packet matches that of the packet which hit the
--set rule. This may be useful if you have problems with people faking their source address in order to DoS you via this
module by disallowing others access to your site by sending bogus packets to you.
Examples:
iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP
iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy --set -j DROP
Steve's ipt_recent website (http://snowman.net/projects/ipt_recent/) also has some examples of usage.
/proc/net/xt_recent/* are the current lists of addresses and information about each entry of each list.
Each file in /proc/net/xt_recent/ can be read from to see the current list or written two using the following commands to modify
the list:
echo +addr >/proc/net/xt_recent/DEFAULT
to add addr to the DEFAULT list
echo -addr >/proc/net/xt_recent/DEFAULT
to remove addr from the DEFAULT list
echo / >/proc/net/xt_recent/DEFAULT
to flush the DEFAULT list (remove all entries).
The module itself accepts parameters, defaults shown:
ip_list_tot=100
Number of addresses remembered per table.
ip_pkt_list_tot=20
Number of packets per address remembered.
ip_list_hash_size=0
Hash table size. 0 means to calculate it based on ip_list_tot, default: 512.
ip_list_perms=0644
Permissions for /proc/net/xt_recent/* files.
ip_list_uid=0
Numerical UID for ownership of /proc/net/xt_recent/* files.
ip_list_gid=0
Numerical GID for ownership of /proc/net/xt_recent/* files.
rpfilter
Performs a reverse path filter test on a packet. If a reply to the packet would be sent via the same interface that the packet
arrived on, the packet will match. Note that, unlike the in-kernel rp_filter, packets protected by IPSec are not treated spe-
cially. Combine this match with the policy match if you want this. Also, packets arriving via the loopback interface are always
permitted. This match can only be used in the PREROUTING chain of the raw or mangle table.
--loose
Used to specifiy that the reverse path filter test should match even if the selected output device is not the expected
one.
--validmark
Also use the packets' nfmark value when performing the reverse path route lookup.
--accept-local
This will permit packets arriving from the network with a source address that is also assigned to the local machine.
--invert This will invert the sense of the match. Instead of matching packets that passed the reverse path filter test,
match those that have failed it.
Example to log and drop packets failing the reverse path filter test:
iptables -t raw -N RPFILTER
iptables -t raw -A RPFILTER -m rpfilter -j RETURN
iptables -t raw -A RPFILTER -m limit --limit 10/minute -j NFLOG --nflog-prefix "rpfilter drop"
iptables -t raw -A RPFILTER -j DROP
iptables -t raw -A PREROUTING -j RPFILTER
Example to drop failed packets, without logging:
iptables -t raw -A RPFILTER -m rpfilter --invert -j DROP
sctp
[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --chunk-types {all|any|only} chunktype[:flags] [...]
The flag letter in upper case indicates that the flag is to match if set, in the lower case indicates to match if unset.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUTDOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK
ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE ASCONF ASCONF_ACK FORWARD_TSN
chunk type available flags
DATA I U B E i u b e
ABORT T t
SHUTDOWN_COMPLETE T t
(lowercase means flag should be "off", uppercase means "on")
Examples:
iptables -A INPUT -p sctp --dport 80 -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA,INIT -j DROP
iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT
set
This module matches IP sets which can be defined by ipset(8).
[!] --match-set setname flag[,flag]...
where flags are the comma separated list of src and/or dst specifications and there can be no more than six of them. Hence
the command
iptables -A FORWARD -m set --match-set test src,dst
will match packets, for which (if the set type is ipportmap) the source address and destination port pair can be found in
the specified set. If the set type of the specified set is single dimension (for example ipmap), then the command will
match packets for which the source address can be found in the specified set.
The option --match-set can be replaced by --set if that does not clash with an option of other extensions.
Use of -m set requires that ipset kernel support is provided, which, for standard kernels, is the case since Linux 2.6.39.
socket
This matches if an open socket can be found by doing a socket lookup on the packet.
--transparent
Ignore non-transparent sockets.
state
This module, when combined with connection tracking, allows access to the connection tracking state for this packet.
[!] --state state
Where state is a comma separated list of the connection states to match. Possible states are INVALID meaning that the
packet could not be identified for some reason which includes running out of memory and ICMP errors which don't correspond
to any known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in
both directions, NEW meaning that the packet has started a new connection, or otherwise associated with a connection which
has not seen packets in both directions, and RELATED meaning that the packet is starting a new connection, but is associ-
ated with an existing connection, such as an FTP data transfer, or an ICMP error. UNTRACKED meaning that the packet is
not tracked at all, which happens if you use the NOTRACK target in raw table.
statistic
This module matches packets based on some statistic condition. It supports two distinct modes settable with the --mode option.
Supported options:
--mode mode
Set the matching mode of the matching rule, supported modes are random and nth.
[!] --probability p
Set the probability for a packet to be randomly matched. It only works with the random mode. p must be within 0.0 and 1.0.
The supported granularity is in 1/2147483648th increments.
[!] --every n
Match one packet every nth packet. It works only with the nth mode (see also the --packet option).
--packet p
Set the initial counter value (0 <= p <= n-1, default 0) for the nth mode.
string
This modules matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.
--algo {bm|kmp}
Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
--from offset
Set the offset from which it starts looking for any matching. If not passed, default is 0.
--to offset
Set the offset up to which should be scanned. That is, byte offset-1 (counting from 0) is the last one that is scanned.
If not passed, default is the packet size.
[!] --string pattern
Matches the given pattern.
[!] --hex-string pattern
Matches the given pattern in hex notation.
tcp
These extensions can be used if `--protocol tcp' is specified. It provides the following options:
[!] --source-port,--sport port[:port]
Source port or port range specification. This can either be a service name or a port number. An inclusive range can also
be specified, using the format first:last. If the first port is omitted, "0" is assumed; if the last is omitted, "65535"
is assumed. If the first port is greater than the second one they will be swapped. The flag --sport is a convenient
alias for this option.
[!] --destination-port,--dport port[:port]
Destination port or port range specification. The flag --dport is a convenient alias for this option.
[!] --tcp-flags mask comp
Match when the TCP flags are as specified. The first argument mask is the flags which we should examine, written as a
comma-separated list, and the second argument comp is a comma-separated list of flags which must be set. Flags are: SYN
ACK FIN RST URG PSH ALL NONE. Hence the command
iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset.
[!] --syn
Only match TCP packets with the SYN bit set and the ACK,RST and FIN bits cleared. Such packets are used to request TCP
connection initiation; for example, blocking such packets coming in an interface will prevent incoming TCP connections,
but outgoing TCP connections will be unaffected. It is equivalent to --tcp-flags SYN,RST,ACK,FIN SYN. If the "!" flag
precedes the "--syn", the sense of the option is inverted.
[!] --tcp-option number
Match if TCP option set.
tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP header. You can only use this on TCP SYN or SYN/ACK packets,
since the MSS is only negotiated during the TCP handshake at connection startup time.
[!] --mss value[:value]
Match a given TCP MSS value or range.
time
This matches if the packet arrival time/date is within a given range. All options are optional, but are ANDed when specified. All
times are interpreted as UTC by default.
--datestart YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
--datestop YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
Only match during the given time, which must be in ISO 8601 "T" notation. The possible time range is 1970-01-01T00:00:00
to 2038-01-19T04:17:07.
If --datestart or --datestop are not specified, it will default to 1970-01-01 and 2038-01-19, respectively.
--timestart hh:mm[:ss]
--timestop hh:mm[:ss]
Only match during the given daytime. The possible time range is 00:00:00 to 23:59:59. Leading zeroes are allowed (e.g.
"06:03") and correctly interpreted as base-10.
[!] --monthdays day[,day...]
Only match on the given days of the month. Possible values are 1 to 31. Note that specifying 31 will of course not match
on months which do not have a 31st day; the same goes for 28- or 29-day February.
[!] --weekdays day[,day...]
Only match on the given weekdays. Possible values are Mon, Tue, Wed, Thu, Fri, Sat, Sun, or values from 1 to 7, respec-
tively. You may also use two-character variants (Mo, Tu, etc.).
--kerneltz
Use the kernel timezone instead of UTC to determine whether a packet meets the time regulations.
About kernel timezones: Linux keeps the system time in UTC, and always does so. On boot, system time is initialized from a ref-
erential time source. Where this time source has no timezone information, such as the x86 CMOS RTC, UTC will be assumed. If the
time source is however not in UTC, userspace should provide the correct system time and timezone to the kernel once it has the
information.
Local time is a feature on top of the (timezone independent) system time. Each process has its own idea of local time, specified
via the TZ environment variable. The kernel also has its own timezone offset variable. The TZ userspace environment variable
specifies how the UTC-based system time is displayed, e.g. when you run date(1), or what you see on your desktop clock. The TZ
string may resolve to different offsets at different dates, which is what enables the automatic time-jumping in userspace. when
DST changes. The kernel's timezone offset variable is used when it has to convert between non-UTC sources, such as FAT filesys-
tems, to UTC (since the latter is what the rest of the system uses).
The caveat with the kernel timezone is that Linux distributions may ignore to set the kernel timezone, and instead only set the
system time. Even if a particular distribution does set the timezone at boot, it is usually does not keep the kernel timezone
offset - which is what changes on DST - up to date. ntpd will not touch the kernel timezone, so running it will not resolve the
issue. As such, one may encounter a timezone that is always +0000, or one that is wrong half of the time of the year. As such,
using --kerneltz is highly discouraged.
EXAMPLES. To match on weekends, use:
-m time --weekdays Sa,Su
Or, to match (once) on a national holiday block:
-m time --datestart 2007-12-24 --datestop 2007-12-27
Since the stop time is actually inclusive, you would need the following stop time to not match the first second of the new day:
-m time --datestart 2007-01-01T17:00 --datestop 2007-01-01T23:59:59
During lunch hour:
-m time --timestart 12:30 --timestop 13:30
The fourth Friday in the month:
-m time --weekdays Fr --monthdays 22,23,24,25,26,27,28
(Note that this exploits a certain mathematical property. It is not possible to say "fourth Thursday OR fourth Friday" in one
rule. It is possible with multiple rules, though.)
tos
This module matches the 8-bit Type of Service field in the IPv4 header (i.e. including the "Precedence" bits) or the (also
8-bit) Priority field in the IPv6 header.
[!] --tos value[/mask]
Matches packets with the given TOS mark value. If a mask is specified, it is logically ANDed with the TOS mark before the
comparison.
[!] --tos symbol
You can specify a symbolic name when using the tos match for IPv4. The list of recognized TOS names can be obtained by
calling iptables with -m tos -h. Note that this implies a mask of 0x3F, i.e. all but the ECN bits.
ttl
This module matches the time to live field in the IP header.
[!] --ttl-eq ttl
Matches the given TTL value.
--ttl-gt ttl
Matches if TTL is greater than the given TTL value.
--ttl-lt ttl
Matches if TTL is less than the given TTL value.
u32
U32 tests whether quantities of up to 4 bytes extracted from a packet have specified values. The specification of what to extract
is general enough to find data at given offsets from tcp headers or payloads.
[!] --u32 tests
The argument amounts to a program in a small language described below.
tests := location "=" value | tests "&&" location "=" value
value := range | value "," range
range := number | number ":" number
a single number, n, is interpreted the same as n:n. n:m is interpreted as the range of numbers >=n and <=m.
location := number | location operator number
operator := "&" | "<>" | "@"
The operators &, <> and && mean the same as in C. The = is really a set membership operator and the value syntax describes a
set. The @ operator is what allows moving to the next header and is described further below.
There are currently some artificial implementation limits on the size of the tests:
* no more than 10 of "=" (and 9 "&&"s) in the u32 argument
* no more than 10 ranges (and 9 commas) per value
* no more than 10 numbers (and 9 operators) per location
To describe the meaning of location, imagine the following machine that interprets it. There are three registers:
A is of type char *, initially the address of the IP header
B and C are unsigned 32 bit integers, initially zero
The instructions are:
number B = number;
C = (*(A+B)<<24) + (*(A+B+1)<<16) + (*(A+B+2)<<8) + *(A+B+3)
&number C = C & number
<< number C = C << number
>> number C = C >> number
@number A = A + C; then do the instruction number
Any access of memory outside [skb->data,skb->end] causes the match to fail. Otherwise the result of the computation is the final
value of C.
Whitespace is allowed but not required in the tests. However, the characters that do occur there are likely to require shell
quoting, so it is a good idea to enclose the arguments in quotes.
Example:
match IP packets with total length >= 256
The IP header contains a total length field in bytes 2-3.
--u32 "0 & 0xFFFF = 0x100:0xFFFF"
read bytes 0-3
AND that with 0xFFFF (giving bytes 2-3), and test whether that is in the range [0x100:0xFFFF]
Example: (more realistic, hence more complicated)
match ICMP packets with icmp type 0
First test that it is an ICMP packet, true iff byte 9 (protocol) = 1
--u32 "6 & 0xFF = 1 && ...
read bytes 6-9, use & to throw away bytes 6-8 and compare the result to 1. Next test that it is not a fragment. (If so, it
might be part of such a packet but we cannot always tell.) N.B.: This test is generally needed if you want to match any-
thing beyond the IP header. The last 6 bits of byte 6 and all of byte 7 are 0 iff this is a complete packet (not a frag-
ment). Alternatively, you can allow first fragments by only testing the last 5 bits of byte 6.
... 4 & 0x3FFF = 0 && ...
Last test: the first byte past the IP header (the type) is 0. This is where we have to use the @syntax. The length of the
IP header (IHL) in 32 bit words is stored in the right half of byte 0 of the IP header itself.
... 0 >> 22 & 0x3C @ 0 >> 24 = 0"
The first 0 means read bytes 0-3, >>22 means shift that 22 bits to the right. Shifting 24 bits would give the first byte,
so only 22 bits is four times that plus a few more bits. &3C then eliminates the two extra bits on the right and the first
four bits of the first byte. For instance, if IHL=5, then the IP header is 20 (4 x 5) bytes long. In this case, bytes 0-1
are (in binary) xxxx0101 yyzzzzzz, >>22 gives the 10 bit value xxxx0101yy and &3C gives 010100. @ means to use this number
as a new offset into the packet, and read four bytes starting from there. This is the first 4 bytes of the ICMP payload,
of which byte 0 is the ICMP type. Therefore, we simply shift the value 24 to the right to throw out all but the first byte
and compare the result with 0.
Example:
TCP payload bytes 8-12 is any of 1, 2, 5 or 8
First we test that the packet is a tcp packet (similar to ICMP).
--u32 "6 & 0xFF = 6 && ...
Next, test that it is not a fragment (same as above).
... 0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 = 1,2,5,8"
0>>22&3C as above computes the number of bytes in the IP header. @ makes this the new offset into the packet, which is the
start of the TCP header. The length of the TCP header (again in 32 bit words) is the left half of byte 12 of the TCP
header. The 12>>26&3C computes this length in bytes (similar to the IP header before). "@" makes this the new offset,
which is the start of the TCP payload. Finally, 8 reads bytes 8-12 of the payload and = checks whether the result is any
of 1, 2, 5 or 8.
udp
These extensions can be used if `--protocol udp' is specified. It provides the following options:
[!] --source-port,--sport port[:port]
Source port or port range specification. See the description of the --source-port option of the TCP extension for
details.
[!] --destination-port,--dport port[:port]
Destination port or port range specification. See the description of the --destination-port option of the TCP extension
for details.
unclean
This module takes no options, but attempts to match packets which seem malformed or unusual. This is regarded as experimental.
TARGET EXTENSIONS
iptables can use extended target modules: the following are included in the standard distribution.
AUDIT
This target allows to create audit records for packets hitting the target. It can be used to record accepted, dropped, and
rejected packets. See auditd(8) for additional details.
--type {accept|drop|reject}
Set type of audit record.
Example:
iptables -N AUDIT_DROP
iptables -A AUDIT_DROP -j AUDIT --type drop
iptables -A AUDIT_DROP -j DROP
CHECKSUM
This target allows to selectively work around broken/old applications. It can only be used in the mangle table.
--checksum-fill
Compute and fill in the checksum in a packet that lacks a checksum. This is particularly useful, if you need to work
around old applications such as dhcp clients, that do not work well with checksum offloads, but don't want to disable
checksum offload in your device.
CLASSIFY
This module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).
--set-class major:minor
Set the major and minor class value. The values are always interpreted as hexadecimal even if no 0x prefix is given.
CLUSTERIP
This module allows you to configure a simple cluster of nodes that share a certain IP and MAC address without an explicit load
balancer in front of them. Connections are statically distributed between the nodes in this cluster.
--new Create a new ClusterIP. You always have to set this on the first rule for a given ClusterIP.
--hashmode mode
Specify the hashing mode. Has to be one of sourceip, sourceip-sourceport, sourceip-sourceport-destport.
--clustermac mac
Specify the ClusterIP MAC address. Has to be a link-layer multicast address
--total-nodes num
Number of total nodes within this cluster.
--local-node num
Local node number within this cluster.
--hash-init rnd
Specify the random seed used for hash initialization.
CONNMARK
This module sets the netfilter mark value associated with a connection. The mark is 32 bits wide.
--set-xmark value[/mask]
Zero out the bits given by mask and XOR value into the ctmark.
--save-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the packet mark (nfmark) to the connection mark (ctmark) using the given masks. The new nfmark value is determined as
follows:
ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask)
i.e. ctmask defines what bits to clear and nfmask what bits of the nfmark to XOR into the ctmark. ctmask and nfmask
default to 0xFFFFFFFF.
--restore-mark [--nfmask nfmask] [--ctmask ctmask]
Copy the connection mark (ctmark) to the packet mark (nfmark) using the given masks. The new ctmark value is determined as
follows:
nfmark = (nfmark & ~nfmask) ^ (ctmark & ctmask);
i.e. nfmask defines what bits to clear and ctmask what bits of the ctmark to XOR into the nfmark. ctmask and nfmask
default to 0xFFFFFFFF.
--restore-mark is only valid in the mangle table.
The following mnemonics are available for --set-xmark:
--and-mark bits
Binary AND the ctmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits is the binary negation of bits.)
--or-mark bits
Binary OR the ctmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits
Binary XOR the ctmark with bits. (Mnemonic for --set-xmark bits/0.)
--set-mark value[/mask]
Set the connection mark. If a mask is specified then only those bits set in the mask are modified.
--save-mark [--mask mask]
Copy the nfmark to the ctmark. If a mask is specified, only those bits are copied.
--restore-mark [--mask mask]
Copy the ctmark to the nfmark. If a mask is specified, only those bits are copied. This is only valid in the mangle table.
CONNSECMARK
This module copies security markings from packets to connections (if unlabeled), and from connections back to packets (also only
if unlabeled). Typically used in conjunction with SECMARK, it is valid in the security table (for backwards compatibility with
older kernels, it is also valid in the mangle table).
--save If the packet has a security marking, copy it to the connection if the connection is not marked.
--restore
If the packet does not have a security marking, and the connection does, copy the security marking from the connection to
the packet.
CT
The CT target allows to set parameters for a packet or its associated connection. The target attaches a "template" connection
tracking entry to the packet, which is then used by the conntrack core when initializing a new ct entry. This target is thus only
valid in the "raw" table.
--notrack
Disables connection tracking for this packet.
--helper name
Use the helper identified by name for the connection. This is more flexible than loading the conntrack helper modules with
preset ports.
--ctevents event[,...]
Only generate the specified conntrack events for this connection. Possible event types are: new, related, destroy, reply,
assured, protoinfo, helper, mark (this refers to the ctmark, not nfmark), natseqinfo, secmark (ctsecmark).
--expevents event[,...]
Only generate the specified expectation events for this connection. Possible event types are: new.
--zone id
Assign this packet to zone id and only have lookups done in that zone. By default, packets have zone 0.
--timeout name
Use the timeout policy identified by name for the connection. This is provides more flexible timeout policy definition
than global timeout values available at /proc/sys/net/netfilter/nf_conntrack_*_timeout_*.
DNAT
This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called
from those chains. It specifies that the destination address of the packet should be modified (and all future packets in this
connection will also be mangled), and rules should cease being examined. It takes one type of option:
--to-destination [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new destination IP address, an inclusive range of IP addresses, and optionally, a port range
(which is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then the destination
port will never be modified. If no IP address is specified then only the destination port will be modified.
In Kernels up to 2.6.10 you can add several --to-destination options. For those kernels, if you specify more than one des-
tination address, either via an address range or multiple --to-destination options, a simple round-robin (one after
another in cycle) load balancing takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don't have the abil-
ity to NAT to multiple ranges anymore.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.22).
--persistent
Gives a client the same source-/destination-address for each connection. This supersedes the SAME target. Support for
persistent mappings is available from 2.6.29-rc2.
DSCP
This target allows to alter the value of the DSCP bits within the TOS header of the IPv4 packet. As this manipulates a packet,
it can only be used in the mangle table.
--set-dscp value
Set the DSCP field to a numerical value (can be decimal or hex)
--set-dscp-class class
Set the DSCP field to a DiffServ class.
ECN
This target allows to selectively work around known ECN blackholes. It can only be used in the mangle table.
--ecn-tcp-remove
Remove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p tcp.
IDLETIMER
This target can be used to identify when interfaces have been idle for a certain period of time. Timers are identified by labels
and are created when a rule is set with a new label. The rules also take a timeout value (in seconds) as an option. If more
than one rule uses the same timer label, the timer will be restarted whenever any of the rules get a hit. One entry for each
timer is created in sysfs. This attribute contains the timer remaining for the timer to expire. The attributes are located
under the xt_idletimer class:
/sys/class/xt_idletimer/timers/
When the timer expires, the target module sends a sysfs notification to the userspace, which can then decide what to do (eg. dis-
connect to save power).
--timeout amount
This is the time in seconds that will trigger the notification.
--label string
This is a unique identifier for the timer. The maximum length for the label string is 27 characters.
LOG
Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print some information on
all matching packets (like most IP header fields) via the kernel log (where it can be read with dmesg or syslogd(8)). This is a
"non-terminating target", i.e. rule traversal continues at the next rule. So if you want to LOG the packets you refuse, use two
separate rules with the same matching criteria, first using target LOG then DROP (or REJECT).
--log-level level
Level of logging (numeric or see syslog.conf(5)).
--log-prefix prefix
Prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs.
--log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is readable by users.
--log-tcp-options
Log options from the TCP packet header.
--log-ip-options
Log options from the IP packet header.
--log-uid
Log the userid of the process which generated the packet.
MARK
This target is used to set the Netfilter mark value associated with the packet. It can, for example, be used in conjunction with
routing based on fwmark (needs iproute2). If you plan on doing so, note that the mark needs to be set in the PREROUTING chain of
the mangle table to affect routing. The mark field is 32 bits wide.
--set-xmark value[/mask]
Zeroes out the bits given by mask and XORs value into the packet mark ("nfmark"). If mask is omitted, 0xFFFFFFFF is
assumed.
--set-mark value[/mask]
Zeroes out the bits given by mask and ORs value into the packet mark. If mask is omitted, 0xFFFFFFFF is assumed.
The following mnemonics are available:
--and-mark bits
Binary AND the nfmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits is the binary negation of bits.)
--or-mark bits
Binary OR the nfmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits
Binary XOR the nfmark with bits. (Mnemonic for --set-xmark bits/0.)
MASQUERADE
This target is only valid in the nat table, in the POSTROUTING chain. It should only be used with dynamically assigned IP
(dialup) connections: if you have a static IP address, you should use the SNAT target. Masquerading is equivalent to specifying
a mapping to the IP address of the interface the packet is going out, but also has the effect that connections are forgotten when
the interface goes down. This is the correct behavior when the next dialup is unlikely to have the same interface address (and
hence any established connections are lost anyway).
--to-ports port[-port]
This specifies a range of source ports to use, overriding the default SNAT source port-selection heuristics (see above).
This is only valid if the rule also specifies -p tcp or -p udp.
--random
Randomize source port mapping If option --random is used then port mapping will be randomized (kernel >= 2.6.21).
MIRROR
This is an experimental demonstration target which inverts the source and destination fields in the IP header and retransmits the
packet. It is only valid in the INPUT, FORWARD and PREROUTING chains, and user-defined chains which are only called from those
chains. Note that the outgoing packets are NOT seen by any packet filtering chains, connection tracking or NAT, to avoid loops
and other problems.
NETMAP
This target allows you to statically map a whole network of addresses onto another network of addresses. It can only be used
from rules in the nat table.
--to address[/mask]
Network address to map to. The resulting address will be constructed in the following way: All 'one' bits in the mask are
filled in from the new `address'. All bits that are zero in the mask are filled in from the original address.
NFLOG
This target provides logging of matching packets. When this target is set for a rule, the Linux kernel will pass the packet to
the loaded logging backend to log the packet. This is usually used in combination with nfnetlink_log as logging backend, which
will multicast the packet through a netlink socket to the specified multicast group. One or more userspace processes may sub-
scribe to the group to receive the packets. Like LOG, this is a non-terminating target, i.e. rule traversal continues at the next
rule.
--nflog-group nlgroup
The netlink group (0 - 2^16-1) to which packets are (only applicable for nfnetlink_log). The default value is 0.
--nflog-prefix prefix
A prefix string to include in the log message, up to 64 characters long, useful for distinguishing messages in the logs.
--nflog-range size
The number of bytes to be copied to userspace (only applicable for nfnetlink_log). nfnetlink_log instances may specify
their own range, this option overrides it.
--nflog-threshold size
Number of packets to queue inside the kernel before sending them to userspace (only applicable for nfnetlink_log). Higher
values result in less overhead per packet, but increase delay until the packets reach userspace. The default value is 1.
NFQUEUE
This target is an extension of the QUEUE target. As opposed to QUEUE, it allows you to put a packet into any specific queue,
identified by its 16-bit queue number. It can only be used with Kernel versions 2.6.14 or later, since it requires the
nfnetlink_queue kernel support. The queue-balance option was added in Linux 2.6.31, queue-bypass in 2.6.39.
--queue-num value
This specifies the QUEUE number to use. Valid queue numbers are 0 to 65535. The default value is 0.
--queue-balance value:value
This specifies a range of queues to use. Packets are then balanced across the given queues. This is useful for multicore
systems: start multiple instances of the userspace program on queues x, x+1, .. x+n and use "--queue-balance x:x+n".
Packets belonging to the same connection are put into the same nfqueue.
--queue-bypass
By default, if no userspace program is listening on an NFQUEUE, then all packets that are to be queued are dropped. When
this option is used, the NFQUEUE rule is silently bypassed instead. The packet will move on to the next rule.
NOTRACK
This target disables connection tracking for all packets matching that rule.
It can only be used in the raw table.
RATEEST
The RATEEST target collects statistics, performs rate estimation calculation and saves the results for later evaluation using the
rateest match.
--rateest-name name
Count matched packets into the pool referred to by name, which is freely choosable.
--rateest-interval amount{s|ms|us}
Rate measurement interval, in seconds, milliseconds or microseconds.
--rateest-ewmalog value
Rate measurement averaging time constant.
REDIRECT
This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called
from those chains. It redirects the packet to the machine itself by changing the destination IP to the primary address of the
incoming interface (locally-generated packets are mapped to the 127.0.0.1 address).
--to-ports port[-port]
This specifies a destination port or range of ports to use: without this, the destination port is never altered. This is
only valid if the rule also specifies -p tcp or -p udp.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.22).
REJECT
This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so it is a termi-
nating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains
which are only called from those chains. The following option controls the nature of the error packet returned:
--reject-with type
The type given can be icmp-net-unreachable, icmp-host-unreachable, icmp-port-unreachable, icmp-proto-unreachable,
icmp-net-prohibited, icmp-host-prohibited or icmp-admin-prohibited (*) which return the appropriate ICMP error message
(port-unreachable is the default). The option tcp-reset can be used on rules which only match the TCP protocol: this
causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur
when sending mail to broken mail hosts (which won't accept your mail otherwise).
(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT
SAME
Similar to SNAT/DNAT depending on chain: it takes a range of addresses (`--to 1.2.3.4-1.2.3.7') and gives a client the same
source-/destination-address for each connection.
N.B.: The DNAT target's --persistent option replaced the SAME target.
--to ipaddr[-ipaddr]
Addresses to map source to. May be specified more than once for multiple ranges.
--nodst
Don't use the destination-ip in the calculations when selecting the new source-ip
--random
Port mapping will be forcibly randomized to avoid attacks based on port prediction (kernel >= 2.6.21).
SECMARK
This is used to set the security mark value associated with the packet for use by security subsystems such as SELinux. It is
valid in the security table (for backwards compatibility with older kernels, it is also valid in the mangle table). The mark is
32 bits wide.
--selctx security_context
SET
This modules adds and/or deletes entries from IP sets which can be defined by ipset(8).
--add-set setname flag[,flag...]
add the address(es)/port(s) of the packet to the sets
--del-set setname flag[,flag...]
delete the address(es)/port(s) of the packet from the sets
where flags are src and/or dst specifications and there can be no more than six of them.
--timeout value
when adding entry, the timeout value to use instead of the default one from the set definition
--exist
when adding entry if it already exists, reset the timeout value to the specified one or to the default from the set defi-
nition
Use of -j SET requires that ipset kernel support is provided, which, for standard kernels, is the case since Linux 2.6.39.
SNAT
This target is only valid in the nat table, in the POSTROUTING chain. It specifies that the source address of the packet should
be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes
one type of option:
--to-source [ipaddr[-ipaddr]][:port[-port]]
which can specify a single new source IP address, an inclusive range of IP addresses, and optionally, a port range (which
is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then source ports below 512
will be mapped to other ports below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and
other ports will be mapped to 1024 or above. Where possible, no port alteration will occur.
In Kernels up to 2.6.10, you can add several --to-source options. For those kernels, if you specify more than one source
address, either via an address range or multiple --to-source options, a simple round-robin (one after another in cycle)
takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don't have the ability to NAT to multiple ranges any-
more.
--random
If option --random is used then port mapping will be randomized (kernel >= 2.6.21).
--persistent
Gives a client the same source-/destination-address for each connection. This supersedes the SAME target. Support for
persistent mappings is available from 2.6.29-rc2.
TCPMSS
This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting
it to your outgoing interface's MTU minus 40 for IPv4 or 60 for IPv6, respectively). Of course, it can only be used in conjunc-
tion with -p tcp.
This target is used to overcome criminally braindead ISPs or servers which block "ICMP Fragmentation Needed" or "ICMPv6 Packet
Too Big" packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines
behind it can never exchange large packets:
1. Web browsers connect, then hang with no data received.
2. Small mail works fine, but large emails hang.
3. ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall configuration like:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value
Explicitly sets MSS option to specified value. If the MSS of the packet is already lower than value, it will not be
increased (from Linux 2.6.25 onwards) to avoid more problems with hosts relying on a proper MSS.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40 for IPv4; -60 for IPv6). This may not function as desired where asymmet-
ric routes with differing path MTU exist -- the kernel uses the path MTU which it would use to send packets from itself to
the source and destination IP addresses. Prior to Linux 2.6.25, only the path MTU to the destination IP address was con-
sidered by this option; subsequent kernels also consider the path MTU to the source IP address.
These options are mutually exclusive.
TCPOPTSTRIP
This target will strip TCP options off a TCP packet. (It will actually replace them by NO-OPs.) As such, you will need to add the
-p tcp parameters.
--strip-options option[,option...]
Strip the given option(s). The options may be specified by TCP option number or by symbolic name. The list of recognized
options can be obtained by calling iptables with -j TCPOPTSTRIP -h.
TEE
The TEE target will clone a packet and redirect this clone to another machine on the local network segment. In other words, the
nexthop must be the target, or you will have to configure the nexthop to forward it further if so desired.
--gateway ipaddr
Send the cloned packet to the host reachable at the given IP address. Use of 0.0.0.0 (for IPv4 packets) or :: (IPv6) is
invalid.
To forward all incoming traffic on eth0 to an Network Layer logging box:
-t mangle -A PREROUTING -i eth0 -j TEE --gateway 2001:db8::1
TOS
This module sets the Type of Service field in the IPv4 header (including the "precedence" bits) or the Priority field in the IPv6
header. Note that TOS shares the same bits as DSCP and ECN. The TOS target is only valid in the mangle table.
--set-tos value[/mask]
Zeroes out the bits given by mask (see NOTE below) and XORs value into the TOS/Priority field. If mask is omitted, 0xFF is
assumed.
--set-tos symbol
You can specify a symbolic name when using the TOS target for IPv4. It implies a mask of 0xFF (see NOTE below). The list
of recognized TOS names can be obtained by calling iptables with -j TOS -h.
The following mnemonics are available:
--and-tos bits
Binary AND the TOS value with bits. (Mnemonic for --set-tos 0/invbits, where invbits is the binary negation of bits. See
NOTE below.)
--or-tos bits
Binary OR the TOS value with bits. (Mnemonic for --set-tos bits/bits. See NOTE below.)
--xor-tos bits
Binary XOR the TOS value with bits. (Mnemonic for --set-tos bits/0. See NOTE below.)
NOTE: In Linux kernels up to and including 2.6.38, with the exception of longterm releases 2.6.32 (>=.42), 2.6.33 (>=.15), and
2.6.35 (>=.14), there is a bug whereby IPv6 TOS mangling does not behave as documented and differs from the IPv4 version. The TOS
mask indicates the bits one wants to zero out, so it needs to be inverted before applying it to the original TOS field. However,
the aformentioned kernels forgo the inversion which breaks --set-tos and its mnemonics.
TPROXY
This target is only valid in the mangle table, in the PREROUTING chain and user-defined chains which are only called from this
chain. It redirects the packet to a local socket without changing the packet header in any way. It can also change the mark value
which can then be used in advanced routing rules. It takes three options:
--on-port port
This specifies a destination port to use. It is a required option, 0 means the new destination port is the same as the
original. This is only valid if the rule also specifies -p tcp or -p udp.
--on-ip address
This specifies a destination address to use. By default the address is the IP address of the incoming interface. This is
only valid if the rule also specifies -p tcp or -p udp.
--tproxy-mark value[/mask]
Marks packets with the given value/mask. The fwmark value set here can be used by advanced routing. (Required for trans-
parent proxying to work: otherwise these packets will get forwarded, which is probably not what you want.)
TRACE
This target marks packets so that the kernel will log every rule which match the packets as those traverse the tables, chains,
rules.
A logging backend, such as ip(6)t_LOG or nfnetlink_log, must be loaded for this to be visible. The packets are logged with the
string prefix: "TRACE: tablename:chainname:type:rulenum " where type can be "rule" for plain rule, "return" for implicit rule at
the end of a user defined chain and "policy" for the policy of the built in chains.
It can only be used in the raw table.
TTL
This is used to modify the IPv4 TTL header field. The TTL field determines how many hops (routers) a packet can traverse until
it's time to live is exceeded.
Setting or incrementing the TTL field can potentially be very dangerous, so it should be avoided at any cost. This target is only
valid in mangle table.
Don't ever set or increment the value on packets that leave your local network!
--ttl-set value
Set the TTL value to `value'.
--ttl-dec value
Decrement the TTL value `value' times.
--ttl-inc value
Increment the TTL value `value' times.
ULOG
This target provides userspace logging of matching packets. When this target is set for a rule, the Linux kernel will multicast
this packet through a netlink socket. One or more userspace processes may then subscribe to various multicast groups and receive
the packets. Like LOG, this is a "non-terminating target", i.e. rule traversal continues at the next rule.
--ulog-nlgroup nlgroup
This specifies the netlink group (1-32) to which the packet is sent. Default value is 1.
--ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32 characters long, and useful for distinguishing messages in the
logs.
--ulog-cprange size
Number of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. Default
is 0.
--ulog-qthreshold size
Number of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets inside the kernel and
transmits them as one netlink multipart message to userspace. Default is 1 (for backwards compatibility).
DIAGNOSTICS
Various error messages are printed to standard error. The exit code is 0 for correct functioning. Errors which appear to be
caused by invalid or abused command line parameters cause an exit code of 2, and other errors cause an exit code of 1.
BUGS
Bugs? What's this? ;-) Well, you might want to have a look at http://bugzilla.netfilter.org/
COMPATIBILITY WITH IPCHAINS
This iptables is very similar to ipchains by Rusty Russell. The main difference is that the chains INPUT and OUTPUT are only
traversed for packets coming into the local host and originating from the local host respectively. Hence every packet only
passes through one of the three chains (except loopback traffic, which involves both INPUT and OUTPUT chains); previously a for-
warded packet would pass through all three.
The other main difference is that -i refers to the input interface; -o refers to the output interface, and both are available for
packets entering the FORWARD chain.
The various forms of NAT have been separated out; iptables is a pure packet filter when using the default `filter' table, with
optional extension modules. This should simplify much of the previous confusion over the combination of IP masquerading and
packet filtering seen previously. So the following options are handled differently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
SEE ALSO
iptables-save(8), iptables-restore(8), ip6tables(8), ip6tables-save(8), ip6tables-restore(8), libipq(3).
The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO
details the extensions that are not in the standard distribution, and the netfilter-hacking-HOWTO details the netfilter inter-
nals.
See http://www.netfilter.org/.
AUTHORS
Rusty Russell originally wrote iptables, in early consultation with Michael Neuling.
Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection framework in iptables, then wrote the mangle
table, the owner match, the mark stuff, and ran around doing cool stuff everywhere.
James Morris wrote the TOS target, and tos match.
Jozsef Kadlecsik wrote the REJECT target.
Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as well as the TTL, DSCP, ECN matches and targets.
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Yasuyuki Kozakai, Jozsef Kadlecsik, Patrick McHardy, James Morris,
Pablo Neira Ayuso, Harald Welte and Rusty Russell.
Man page originally written by Herve Eychenne .
VERSION
This manual page applies to iptables @PACKAGE_VERSION@.